diff --git a/nitrogen/CHANGELOG.md b/nitrogen/CHANGELOG.md
index 6f6c7e5..0ddd2b3 100644
--- a/nitrogen/CHANGELOG.md
+++ b/nitrogen/CHANGELOG.md
@@ -1,3 +1,8 @@
+## 0.3.0
+* Add configuration for output locations.
+* Move configuration from `pubspec.yaml` to `build.yaml`.
+* Remove generation of `flutter_extension`.
+
 ## 0.2.0
 * Add `flutter-extension` option.
 * Change `file` asset key option to `file-name`.
diff --git a/nitrogen/README.md b/nitrogen/README.md
index ff95ece..90c12c5 100644
--- a/nitrogen/README.md
+++ b/nitrogen/README.md
@@ -36,13 +36,13 @@ Widget build(BuildContext context) => Assets.images.foo();
 
 ## Getting started
 
-3rd party packages are supported via 'extension' packages. `extension` packages generate `extension`s in separate files.
-These `extension`s provide a `call(...)` function that transforms an `Asset` into a 3rd party type.
+3rd party packages are supported via 'extension' packages. `extension` packages contain an `extension` that provide a 
+`call(...)` function that transforms an `Asset` into a 3rd party type.
 
-| Type              | Package       | Extension Package      | Version                                                                                                        | Default generated file           |
-|-------------------|---------------|------------------------|----------------------------------------------------------------------------------------------------------------|----------------------------------|
-| SVG images        | `flutter_svg` | `nitrogen_flutter_svg` | [![Pub Dev](https://img.shields.io/pub/v/nitrogen_flutter_svg)](https://pub.dev/packages/nitrogen_flutter_svg) | `svg_extension.nitrogen.dart`    |
-| Lottie animations | `lottie`      | `nitrogen_lottie`      | [![Pub Dev](https://img.shields.io/pub/v/nitrogen_lottie)](https://pub.dev/packages/nitrogen_lottie)           | `lottie_extension.nitrogen.dart` |
+| Type              | Package       | Extension Package      | Version                                                                                                        |
+|-------------------|---------------|------------------------|----------------------------------------------------------------------------------------------------------------|
+| SVG images        | `flutter_svg` | `nitrogen_flutter_svg` | [![Pub Dev](https://img.shields.io/pub/v/nitrogen_flutter_svg)](https://pub.dev/packages/nitrogen_flutter_svg) |
+| Lottie animations | `lottie`      | `nitrogen_lottie`      | [![Pub Dev](https://img.shields.io/pub/v/nitrogen_lottie)](https://pub.dev/packages/nitrogen_lottie)           |
 
 
 Install the following:
@@ -72,19 +72,26 @@ dart run build_runner build
 
 ## Configuration
 
-Nitrogen's configuration can be set in the `nitrogen` section of your pubspec.yaml. In most cases, the default
+Nitrogen's configuration can be set in the `nitrogen` section of your build.yaml. In most cases, the default
 configuration works out of the box.
 
 ### Example
 
 A simple configuration looks like:
 ```yaml
-nitrogen:
-  package: true
-  prefix: 'MyPrefix'
-  key: file
-  themes:
-    fallback: assets/themes/light
+targets:
+  $default:
+    builders:
+      nitrogen:
+        options:
+          package: true
+          prefix: 'MyPrefix'
+          key: file-name
+          assets:
+            output: 'lib/src/assets.nitrogen.dart'
+          themes:
+            output: 'lib/src/asset_themes.nitrogen.dart'
+            fallback: assets/themes/light
 ```
 
 ### `package`
@@ -115,6 +122,15 @@ Optional. Defaults to `file`. Controls the generated assets' key parameters. The
 | file-name | file name, without the extension                      | `assets/images/foo.png` | `foo`         |
 | grpc-enum | parent directory and file name, without the extension | `assets/images/foo.png` | `IMAGES_FOO`  |
 
+### `assets`
+
+Optional. Controls the location of the generated assets file.
+
+```yaml
+assets:
+  # Path to the generated assets file. Relative to the project root. Defaults to `lib/src/assets.nitrogen.dart`.
+  output: lib/src/my/path/assets.nitrogen.dart
+``` 
 
 ### `themes`
 
@@ -122,8 +138,9 @@ Optional. Defaults to `null`. Controls whether to generate an additional `asset_
 working with theme-specific assets.
 
 ```yaml
-nitrogen:
-  themes:
-    # Path to fallback theme, relative to package root. Assumes that all themes are under 'assets/themes'.
-    fallback: assets/themes/light
+themes:
+  # Path to the generated themes file. Relative to the project root. Defaults to `lib/src/asset_themes.nitrogen.dart`.
+  output: lib/src/my/path/assets_themes.nitrogen.dart
+  # Path to fallback theme, relative to package root. Assumes that all themes are under 'assets/themes'.
+  fallback: assets/themes/light
 ```
diff --git a/nitrogen/build.yaml b/nitrogen/build.yaml
index 3cf1f7e..5207ac9 100644
--- a/nitrogen/build.yaml
+++ b/nitrogen/build.yaml
@@ -12,6 +12,15 @@ builders:
   nitrogen:
     import: "package:nitrogen/nitrogen.dart"
     builder_factories: [ "nitrogenBuilder" ]
-    build_extensions: { "$package$": [ "lib/src/assets.nitrogen.dart", "lib/src/asset_themes.nitrogen.dart", "lib/src/flutter_extension.nitrogen.dart"] }
+    build_extensions: { "$package$": [ "lib/src/assets.nitrogen.dart", "lib/src/asset_themes.nitrogen.dart" ] }
+    defaults:
+      options:
+        package: false
+        prefix: ""
+        key: "file-name"
+        assets:
+          output: "lib/src/assets.nitrogen.dart"
+        themes:
+          output: "lib/src/asset_themes.nitrogen.dart"
     auto_apply: dependents
     build_to: source
\ No newline at end of file
diff --git a/nitrogen/lib/nitrogen.dart b/nitrogen/lib/nitrogen.dart
index 9068afe..780b69c 100644
--- a/nitrogen/lib/nitrogen.dart
+++ b/nitrogen/lib/nitrogen.dart
@@ -2,6 +2,10 @@ import 'dart:async';
 
 import 'package:build/build.dart';
 import 'package:glob/glob.dart';
+import 'package:path/path.dart';
+import 'package:yaml/yaml.dart';
+
+import 'package:nitrogen/src/configuration/build_configuration.dart';
 import 'package:nitrogen/src/configuration/configuration.dart';
 import 'package:nitrogen/src/file_system.dart';
 import 'package:nitrogen/src/generators/asset_generator.dart';
@@ -9,25 +13,34 @@ import 'package:nitrogen/src/generators/theme_generator.dart';
 import 'package:nitrogen/src/lints/reserved_keyword_lint.dart';
 import 'package:nitrogen/src/nitrogen_exception.dart';
 import 'package:nitrogen/src/walker.dart';
-import 'package:path/path.dart';
-import 'package:yaml/yaml.dart';
 
 /// Creates a [NitrogenBuilder].
-Builder nitrogenBuilder(BuilderOptions options) => NitrogenBuilder();
+Builder nitrogenBuilder(BuilderOptions options) {
+  try {
+    BuildConfiguration.lint(options.config);
+    return NitrogenBuilder(BuildConfiguration.parse(options.config));
+
+  } on NitrogenException {
+    return EmptyBuilder();
+  }
+}
 
 /// A Nitrogen builder.
 class NitrogenBuilder extends Builder {
 
   static final _pubspec = Glob('pubspec.yaml');
   static final _assets = Glob('assets/**');
+
+  final BuildConfiguration _configuration;
+
+  /// Creates a [NitrogenBuilder].
+  NitrogenBuilder(this._configuration);
   
   @override
   FutureOr<void> build(BuildStep buildStep) async {
     try {
       final pubspec = loadYaml(await buildStep.readAsString(await buildStep.findAssets(_pubspec).first));
-
-      Configuration.lint(pubspec);
-      final configuration = Configuration.parse(pubspec);
+      final configuration = Configuration.merge(_configuration, pubspec);
 
       final walker = Walker(configuration.package, configuration.flutterAssets, configuration.key.call);
 
@@ -38,8 +51,8 @@ class NitrogenBuilder extends Builder {
 
       lintReservedKeyword(assets);
 
-      final assetsOutput = AssetId(buildStep.inputId.package, join('lib', 'src', 'assets.nitrogen.dart'));
-      if (configuration.fallbackTheme case final fallback?) {
+      final assetsOutput = AssetId(buildStep.inputId.package, configuration.assets.output);
+      if (configuration.themes case (:final fallback, :final output)) {
         var themes = assets;
         var fallbackTheme = assets;
         for (final segment in split(fallback).skip(1)) {
@@ -53,7 +66,7 @@ class NitrogenBuilder extends Builder {
         );
 
         await buildStep.writeAsString(
-          AssetId(buildStep.inputId.package, join('lib', 'src', 'asset_themes.nitrogen.dart')),
+          AssetId(buildStep.inputId.package, output),
           ThemeGenerator(configuration.prefix, themes, fallbackTheme).generate(),
         );
 
@@ -72,9 +85,19 @@ class NitrogenBuilder extends Builder {
   @override
   Map<String, List<String>> get buildExtensions => {
     r'$package$': [
-      'lib/src/assets.nitrogen.dart',
-      'lib/src/asset_themes.nitrogen.dart',
+      _configuration.assets.output,
+      if (_configuration.themes != null)
+        _configuration.themes!.output,
     ],
   };
   
 }
+
+/// A stub builder for when Nitrogen fails to build.
+class EmptyBuilder extends Builder {
+  @override
+  void build(BuildStep buildStep) {}
+
+  @override
+  Map<String, List<String>> get buildExtensions => {};
+}
diff --git a/nitrogen/lib/src/configuration/build_configuration.dart b/nitrogen/lib/src/configuration/build_configuration.dart
new file mode 100644
index 0000000..46e870f
--- /dev/null
+++ b/nitrogen/lib/src/configuration/build_configuration.dart
@@ -0,0 +1,92 @@
+import 'package:build/build.dart';
+import 'package:meta/meta.dart';
+
+import 'package:nitrogen/src/configuration/key.dart';
+import 'package:nitrogen/src/nitrogen_exception.dart';
+
+ /// Nitrogen's build.yaml configuration.
+final class BuildConfiguration {
+
+  /// The Nitrogen configuration's valid keys.
+  static const keys = { 'package', 'prefix', 'key', 'assets', 'themes' };
+
+  /// Lints the pubspec.
+  static void lint(Map<dynamic, dynamic> configuration) {
+    for (final key in {...configuration.keys }..removeAll(keys)) {
+      log.warning('Unknown key, "$key", in build.yaml\'s nitrogen configuration. See https://github.com/forus-labs/cauldron/tree/master/nitrogen#configuration for valid configuration options.');
+    }
+
+    final assets = configuration['assets'] as Map;
+    for (final key in {...assets.keys }..removeAll({ 'output' })) {
+      log.warning('Unknown key, "$key", in build.yaml\'s nitrogen assets configuration. See https://github.com/forus-labs/cauldron/tree/master/nitrogen#assets for valid configuration options.');
+    }
+
+    final themes = configuration['themes'] as Map;
+    for (final key in {...themes.keys }..removeAll({ 'fallback', 'output' })) {
+      log.warning('Unknown key, "$key", in build.yaml\'s nitrogen themes configuration. See https://github.com/forus-labs/cauldron/tree/master/nitrogen#themes for valid configuration options.');
+    }
+  }
+
+  /// Parses the assets configuration.
+  @visibleForTesting
+  static ({String output,}) parseAssets(Map<dynamic, dynamic> configuration) {
+    switch (configuration) {
+      case { 'output': final String output }:
+        return (output: output);
+
+      default:
+        log.severe("Unable to read assets configuration in build.yaml's nitrogen configuration. See https://github.com/forus-labs/cauldron/tree/master/nitrogen#assets.");
+        throw NitrogenException();
+    }
+  }
+
+  /// Parses the themes configuration.
+  @visibleForTesting
+  static ({String fallback, String output})? parseThemes(Map<dynamic, dynamic> configuration) {
+    switch (configuration) {
+      case { 'fallback': final String fallback, 'output': final String output }:
+        return (fallback: fallback, output: output);
+
+      case { 'output': String _ }:
+        return null;
+
+      default:
+        log.severe("Unable to read themes configuration in build.yaml's nitrogen configuration. See https://github.com/forus-labs/cauldron/tree/master/nitrogen#themes.");
+        throw NitrogenException();
+    }
+  }
+
+  /// Whether to generate assets for a package.
+  final bool package;
+
+  /// The prefix for generated classes.
+  final String prefix;
+
+  /// The function for generating asset keys.
+  final String Function(List<String>) key;
+
+  /// The output location of the generated assets.
+  final ({String output,}) assets;
+
+  /// The fallback theme and output location of the generated themes.
+  final ({String fallback, String output})? themes;
+
+  /// Parses the build configuration.
+  factory BuildConfiguration.parse(Map<dynamic, dynamic> configuration) => BuildConfiguration(
+    package: configuration['package'],
+    prefix: configuration['prefix'],
+    key: Key.parse(configuration['key']),
+    assets: parseAssets(configuration['assets']),
+    themes: parseThemes(configuration['themes']),
+  );
+
+  /// Creates a [BuildConfiguration].
+  BuildConfiguration({
+    required this.package,
+    required this.prefix,
+    required this.key,
+    required this.assets,
+    required this.themes,
+  });
+
+}
diff --git a/nitrogen/lib/src/configuration/configuration.dart b/nitrogen/lib/src/configuration/configuration.dart
index ceb4aab..0f3b7c1 100644
--- a/nitrogen/lib/src/configuration/configuration.dart
+++ b/nitrogen/lib/src/configuration/configuration.dart
@@ -1,46 +1,27 @@
 import 'package:build/build.dart';
 import 'package:meta/meta.dart';
-import 'package:nitrogen/src/nitrogen_exception.dart';
-import 'package:nitrogen/src/configuration/key.dart';
 import 'package:yaml/yaml.dart';
 
+import 'package:nitrogen/src/configuration/build_configuration.dart';
+import 'package:nitrogen/src/nitrogen_exception.dart';
+
 /// Nitrogen's configuration.
 final class Configuration {
 
-  /// The Nitrogen section's valid keys.
-  static const keys = { 'package', 'prefix', 'key', 'themes' };
-
-  /// Lints the pubspec.
-  static void lint(YamlMap? pubspec) {
-    final nitrogen = pubspec?.nodes['nitrogen'].as<YamlMap>();
-
-    for (final key in {...?nitrogen?.keys }..removeAll(keys)) {
-      log.warning(nitrogen?.nodes[key]!.span.message('Unknown key, "$key", in pubspec.yaml\'s nitrogen section. See https://github.com/forus-labs/cauldron/tree/master/nitrogen#configuration for valid configuration options.'));
-    }
-
-    final themes = nitrogen?.nodes['themes'].as<YamlMap>();
-    for (final key in {...?themes?.keys }..removeAll({ 'fallback' })) {
-      log.warning(themes?.nodes[key]!.span.message('Unknown key, "$key", in pubspec.yaml\'s nitrogen section. See https://github.com/forus-labs/cauldron/tree/master/nitrogen#themes for valid configuration options.'));
-    }
-  }
-
-
   /// Parses the configuration from the project's pubspec.yaml.
-  factory Configuration.parse(YamlMap pubspec) {
-    final nitrogen = pubspec.nodes['nitrogen'].as<YamlMap>();
-    return Configuration(
-      package: parsePackage(pubspec.nodes['name'], nitrogen?.nodes['package']),
-      prefix: parsePrefix(nitrogen?.nodes['prefix']),
-      key: Key.parse(nitrogen?.nodes['key']),
-      fallbackTheme: parseFallbackTheme(nitrogen?.nodes['themes']),
-      flutterAssets: parseFlutterAssets(pubspec.nodes['flutter'].as<YamlMap>()?.nodes['assets']),
-    );
-  }
+  factory Configuration.merge(BuildConfiguration configuration, YamlMap pubspec) => Configuration(
+    package: parsePackage(pubspec.nodes['name'], enabled: configuration.package),
+    prefix: configuration.prefix,
+    key: configuration.key,
+    assets: configuration.assets,
+    themes: configuration.themes,
+    flutterAssets: parseFlutterAssets(pubspec.nodes['flutter'].as<YamlMap>()?.nodes['assets']),
+  );
 
   /// Parses the package name from the project's pubspec.yaml.
   @visibleForTesting
-  static String? parsePackage(YamlNode? name, YamlNode? enabled) {
-    switch ((name?.value, enabled?.value)) {
+  static String? parsePackage(YamlNode? name, {required bool enabled}) {
+    switch ((name?.value, enabled)) {
       case (final String name, true):
         return name;
 
@@ -50,39 +31,6 @@ final class Configuration {
 
       case (_, bool? _):
         return null;
-
-      default:
-        log.severe(enabled?.span.message('Unable to read package name. See https://github.com/forus-labs/cauldron/tree/master/nitrogen#package.'));
-        throw NitrogenException();
-    }
-  }
-
-  /// Parses the class prefix from the nitrogen section in the project's pubspec.yaml.
-  @visibleForTesting
-  static String parsePrefix(YamlNode? node) {
-    switch (node?.value) {
-      case final String? prefix:
-        return prefix ?? '';
-
-      default:
-        log.severe(node!.span.message('Unable to read prefix. See https://github.com/forus-labs/cauldron/tree/master/nitrogen#prefix.'));
-        throw NitrogenException();
-    }
-  }
-
-  /// Parses the path to the fallback theme.
-  @visibleForTesting
-  static String? parseFallbackTheme(YamlNode? node) {
-    switch (node?.value) {
-      case null:
-        return null;
-
-      case { 'fallback': final String fallback }:
-        return fallback;
-
-      case _:
-        log.severe(node!.span.message('Unable to read themes. See https://github.com/forus-labs/cauldron/tree/master/nitrogen#themes.'));
-        throw NitrogenException();
     }
   }
 
@@ -111,8 +59,11 @@ final class Configuration {
   /// The function for generating asset keys.
   final String Function(List<String>) key;
 
-  /// The path to the fallback theme.
-  final String? fallbackTheme;
+  /// The output location of the generated assets.
+  final ({String output,}) assets;
+
+  /// The fallback theme and output location of the generated themes.
+  final ({String fallback, String output})? themes;
 
   /// The flutter assets.
   final Set<String> flutterAssets;
@@ -122,7 +73,8 @@ final class Configuration {
     required this.package,
     required this.prefix,
     required this.key,
-    required this.fallbackTheme,
+    required this.assets,
+    required this.themes,
     required this.flutterAssets,
   });
 
diff --git a/nitrogen/lib/src/configuration/key.dart b/nitrogen/lib/src/configuration/key.dart
index 2dec369..0b773eb 100644
--- a/nitrogen/lib/src/configuration/key.dart
+++ b/nitrogen/lib/src/configuration/key.dart
@@ -1,23 +1,23 @@
 import 'package:build/build.dart';
-import 'package:nitrogen/src/nitrogen_exception.dart';
 import 'package:path/path.dart';
 import 'package:sugar/sugar.dart';
-import 'package:yaml/yaml.dart';
+
+import 'package:nitrogen/src/nitrogen_exception.dart';
 
 /// Provides functions for generating asset keys.
 extension Key on Never {
 
   /// Parses the `asset-key` node if valid.
-  static String Function(List<String>) parse(YamlNode? key) {
-    switch (key?.value) {
+  static String Function(List<String>) parse(String key) {
+    switch (key) {
       case 'grpc-enum':
         return grpcEnum;
 
-      case 'file-name' || null:
+      case 'file-name':
         return fileName;
 
       default:
-        log.severe(key!.span.message('Unable to read asset key. See https://github.com/forus-labs/cauldron/tree/master/nitrogen#key.'));
+        log.severe('Unknown asset key: "$key". See https://github.com/forus-labs/cauldron/tree/master/nitrogen#key.');
         throw NitrogenException();
     }
   }
diff --git a/nitrogen/lib/src/generators/asset_generator.dart b/nitrogen/lib/src/generators/asset_generator.dart
index 253b719..1094d6a 100644
--- a/nitrogen/lib/src/generators/asset_generator.dart
+++ b/nitrogen/lib/src/generators/asset_generator.dart
@@ -1,9 +1,9 @@
 import 'package:code_builder/code_builder.dart';
-import 'package:nitrogen/src/file_system.dart';
-import 'package:nitrogen/src/libraries.dart';
 import 'package:nitrogen_types/nitrogen_types.dart';
 import 'package:sugar/sugar.dart';
 
+import 'package:nitrogen/src/file_system.dart';
+import 'package:nitrogen/src/libraries.dart';
 
 /// A generator for standard class representations of asset directories.
 class AssetGenerator {
diff --git a/nitrogen/lib/src/generators/theme_generator.dart b/nitrogen/lib/src/generators/theme_generator.dart
index 428f8ec..c170111 100644
--- a/nitrogen/lib/src/generators/theme_generator.dart
+++ b/nitrogen/lib/src/generators/theme_generator.dart
@@ -1,11 +1,11 @@
 import 'dart:collection';
 
 import 'package:code_builder/code_builder.dart';
+import 'package:sugar/sugar.dart';
+
 import 'package:nitrogen/src/file_system.dart';
 import 'package:nitrogen/src/generators/asset_generator.dart';
 import 'package:nitrogen/src/libraries.dart';
-import 'package:sugar/sugar.dart';
-
 
 /// A generator for theme class representations of asset directories.
 class ThemeGenerator {
diff --git a/nitrogen/lib/src/lints/reserved_keyword_lint.dart b/nitrogen/lib/src/lints/reserved_keyword_lint.dart
index 8990fa1..0ddd03e 100644
--- a/nitrogen/lib/src/lints/reserved_keyword_lint.dart
+++ b/nitrogen/lib/src/lints/reserved_keyword_lint.dart
@@ -1,4 +1,5 @@
 import 'package:build/build.dart';
+
 import 'package:nitrogen/src/file_system.dart';
 import 'package:nitrogen/src/nitrogen_exception.dart';
 
diff --git a/nitrogen/lib/src/walker.dart b/nitrogen/lib/src/walker.dart
index f758a29..d371d27 100644
--- a/nitrogen/lib/src/walker.dart
+++ b/nitrogen/lib/src/walker.dart
@@ -1,9 +1,10 @@
 import 'dart:collection';
 
 import 'package:build/build.dart';
-import 'package:nitrogen/src/file_system.dart';
 import 'package:nitrogen_types/nitrogen_types.dart';
 
+import 'package:nitrogen/src/file_system.dart';
+
 /// A walker that recursively walks through a directory and converts all entities into [Entity]s.
 final class Walker {
 
diff --git a/nitrogen/pubspec.yaml b/nitrogen/pubspec.yaml
index d5714e4..653ae12 100644
--- a/nitrogen/pubspec.yaml
+++ b/nitrogen/pubspec.yaml
@@ -27,8 +27,12 @@ dependencies:
 dev_dependencies:
   build_test: ^2.2.2
   flint: ^2.7.0
+  import_sorter: ^4.6.0
   test: ^1.25.2
 
 dependency_overrides:
   nitrogen_types:
     path: ../nitrogen_types
+
+import_sorter:
+  comments: false
diff --git a/nitrogen/test/src/configuration/build_configuration_test.dart b/nitrogen/test/src/configuration/build_configuration_test.dart
new file mode 100644
index 0000000..a0ac110
--- /dev/null
+++ b/nitrogen/test/src/configuration/build_configuration_test.dart
@@ -0,0 +1,93 @@
+import 'package:build/build.dart';
+import 'package:build_test/build_test.dart';
+import 'package:test/test.dart';
+import 'package:yaml/yaml.dart';
+
+import 'package:nitrogen/src/configuration/build_configuration.dart';
+import 'package:nitrogen/src/configuration/key.dart';
+import 'package:nitrogen/src/nitrogen_exception.dart';
+
+const valid = '''
+package: true
+prefix: 'MyPrefix'
+key: grpc-enum
+assets:
+  output: foo-output
+themes:
+  fallback: assets/path/to/first/fallback
+  output: bar-output
+''';
+
+const invalid = '''
+package: true
+prefix: 'MyPrefix'
+key: grpc-enum
+invalid-nitrogen: true
+assets:
+  invalid-assets: true
+themes:
+  fallback: true
+  invalid-theme: true
+''';
+
+void main() {
+  group('lint(...)', () {
+    test('invalid pubspec', () {
+      expectLater(
+        log.onRecord,
+        emitsInOrder([
+          warningLogOf(contains('Unknown key, "invalid-nitrogen", in build.yaml\'s nitrogen configuration. See https://github.com/forus-labs/cauldron/tree/master/nitrogen#configuration for valid configuration options.')),
+          warningLogOf(contains('Unknown key, "invalid-assets", in build.yaml\'s nitrogen assets configuration. See https://github.com/forus-labs/cauldron/tree/master/nitrogen#assets for valid configuration options.')),
+          warningLogOf(contains('Unknown key, "invalid-theme", in build.yaml\'s nitrogen themes configuration. See https://github.com/forus-labs/cauldron/tree/master/nitrogen#themes for valid configuration options.')),
+        ]),
+      );
+
+      BuildConfiguration.lint(loadYaml(invalid));
+    });
+  });
+
+  group('parseAssets(...)', () {
+    test('valid configuration', () => expect(BuildConfiguration.parseAssets({ 'output': 'valid' }).output, 'valid'));
+
+    test('invalid configuration', () {
+      expectLater(
+        log.onRecord,
+        emits(severeLogOf(contains("Unable to read assets configuration in build.yaml's nitrogen configuration. See https://github.com/forus-labs/cauldron/tree/master/nitrogen#assets."))),
+      );
+
+      expect(() => BuildConfiguration.parseAssets({}), throwsA(isA<NitrogenException>()));
+    });
+  });
+
+  group('parseThemes(...)', () {
+    test('valid configuration', () {
+      final configuration = BuildConfiguration.parseThemes({ 'fallback': 'valid-fallback', 'output': 'valid-output' });
+      expect(configuration?.fallback, 'valid-fallback');
+      expect(configuration?.output, 'valid-output');
+    });
+
+    test('no fallback', () => expect(BuildConfiguration.parseThemes({ 'output': 'valid-output' }), null));
+
+    test('invalid configuration', () {
+      expectLater(
+        log.onRecord,
+        emits(severeLogOf(contains("Unable to read themes configuration in build.yaml's nitrogen configuration. See https://github.com/forus-labs/cauldron/tree/master/nitrogen#themes."))),
+      );
+
+      expect(() => BuildConfiguration.parseThemes({}), throwsA(isA<NitrogenException>()));
+    });
+  });
+
+  group('parse(...)', () {
+    test('valid configuration', () {
+      final configuration = BuildConfiguration.parse(loadYaml(valid));
+
+      expect(configuration.package, true);
+      expect(configuration.prefix, 'MyPrefix');
+      expect(configuration.key, Key.grpcEnum);
+      expect(configuration.assets.output, 'foo-output');
+      expect(configuration.themes?.fallback, 'assets/path/to/first/fallback');
+      expect(configuration.themes?.output, 'bar-output');
+    });
+  });
+}
diff --git a/nitrogen/test/src/configuration/configuration_test.dart b/nitrogen/test/src/configuration/configuration_test.dart
index fcca49d..6549e0f 100644
--- a/nitrogen/test/src/configuration/configuration_test.dart
+++ b/nitrogen/test/src/configuration/configuration_test.dart
@@ -1,46 +1,17 @@
 import 'package:build/build.dart';
 import 'package:build_test/build_test.dart';
+import 'package:test/test.dart';
+import 'package:yaml/yaml.dart';
+
+import 'package:nitrogen/src/configuration/build_configuration.dart';
 import 'package:nitrogen/src/configuration/configuration.dart';
 import 'package:nitrogen/src/configuration/key.dart';
 import 'package:nitrogen/src/nitrogen_exception.dart';
-import 'package:test/test.dart';
-import 'package:yaml/yaml.dart';
 
 // ignore_for_file: avoid_dynamic_calls
 
-const _invalid = '''
-name: hi
-flutter:
-  assets:
-    - path/to/first/
-    - path/to/second/
-nitrogen:
-  package: true
-  prefix: 'MyPrefix'
-  key: grpc-enum
-  invalid-nitrogen: true
-  themes:
-    fallback: true
-    invalid-theme: true
-''';
-
-
 const _pubspec = '''
 name: hi
-flutter:
-  assets:
-    - path/to/first/
-    - path/to/second/
-nitrogen:
-  package: true
-  prefix: 'MyPrefix'
-  key: grpc-enum
-  themes:
-    fallback: assets/path/to/first/fallback
-''';
-
-const _noNitrogen = '''
-name: hi
 flutter:
   assets:
     - path/to/first/
@@ -54,44 +25,29 @@ assets:
 ''';
 
 void main() {
-  group('lint(...)', () {
-    test('invalid pubspec', () {
-      expectLater(
-        log.onRecord,
-        emitsInOrder([
-          warningLogOf(contains('Unknown key, "invalid-nitrogen", in pubspec.yaml\'s nitrogen section. See https://github.com/forus-labs/cauldron/tree/master/nitrogen#configuration for valid configuration options.')),
-          warningLogOf(contains('Unknown key, "invalid-theme", in pubspec.yaml\'s nitrogen section. See https://github.com/forus-labs/cauldron/tree/master/nitrogen#themes for valid configuration options.')),
-        ]),
-      );
-
-      Configuration.lint(loadYaml(_invalid));
-    });
-  });
-
-  group('parse(...)', () {
+  final buildConfiguration = BuildConfiguration(
+    package: true,
+    prefix: 'MyPrefix',
+    key: Key.grpcEnum,
+    assets: (output: 'foo-output'),
+    themes: (fallback: 'some-path', output: 'bar-output'),
+  );
+
+  group('merge(...)', () {
     test('valid pubspec', () {
-      final configuration = Configuration.parse(loadYaml(_pubspec));
+      final configuration = Configuration.merge(buildConfiguration, loadYaml(_pubspec));
 
       expect(configuration.package, 'hi');
-      expect(configuration.prefix, 'MyPrefix');
-      expect(configuration.key, Key.grpcEnum);
-      expect(configuration.fallbackTheme, 'assets/path/to/first/fallback');
-      expect(configuration.flutterAssets, { 'path/to/first/', 'path/to/second/' });
-    });
-
-    test('no nitrogen section', () {
-      final configuration = Configuration.parse(loadYaml(_noNitrogen));
-
-      expect(configuration.package, null);
-      expect(configuration.prefix, '');
-      expect(configuration.key, Key.fileName);
-      expect(configuration.fallbackTheme, null);
+      expect(configuration.prefix, buildConfiguration.prefix);
+      expect(configuration.key, buildConfiguration.key);
+      expect(configuration.assets, buildConfiguration.assets);
+      expect(configuration.themes, buildConfiguration.themes);
       expect(configuration.flutterAssets, { 'path/to/first/', 'path/to/second/' });
     });
   });
 
   group('parsePackage(...)', () {
-    test('use package name', () => expect(Configuration.parsePackage(loadYamlNode('nitrogen'), loadYamlNode('true')), 'nitrogen'));
+    test('use package name', () => expect(Configuration.parsePackage(loadYamlNode('nitrogen'), enabled: true), 'nitrogen'));
 
     test('use package name & invalid name', () {
       expectLater(
@@ -99,53 +55,12 @@ void main() {
         emits(severeLogOf(contains('Unable to read package name in pubspec.yaml. See https://dart.dev/tools/pub/pubspec#name.'))),
       );
       expect(
-        () => Configuration.parsePackage(loadYamlNode('true'), loadYamlNode('true')),
+        () => Configuration.parsePackage(loadYamlNode('true'), enabled: true),
         throwsA(isA<NitrogenException>()),
       );
     });
 
-    test('do not use package name & invalid name', () => expect(Configuration.parsePackage(loadYamlNode('true'), loadYamlNode('false')), null));
-
-    test('null & invalid name', () => expect(Configuration.parsePackage(loadYamlNode('true'), null), null));
-
-    test('invalid package usage', () {
-      expectLater(
-        log.onRecord,
-        emits(severeLogOf(contains('Unable to read package name. See https://github.com/forus-labs/cauldron/tree/master/nitrogen#package.'))),
-      );
-      expect(
-        () => Configuration.parsePackage(loadYamlNode('nitrogen'), loadYamlNode('invalid')),
-        throwsA(isA<NitrogenException>()),
-      );
-    });
-  });
-
-  group('parsePrefix(...)', () {
-    test('prefix', () => expect(Configuration.parsePrefix(loadYamlNode('something')), 'something'));
-
-    test('null', () => expect(Configuration.parsePrefix(null), ''));
-
-    test('invalid', () {
-      expectLater(
-        log.onRecord,
-        emits(severeLogOf(contains('Unable to read prefix. See https://github.com/forus-labs/cauldron/tree/master/nitrogen#prefix.'))),
-      );
-      expect(() => Configuration.parsePrefix(loadYamlNode('true')), throwsA(isA<NitrogenException>()));
-    });
-  });
-
-  group('parseFallbackTheme(...)', () {
-    test('fallback theme', () => expect(Configuration.parseFallbackTheme(loadYamlNode('fallback: assets/path/to/fallback')), 'assets/path/to/fallback'));
-
-    test('null', () => expect(Configuration.parseFallbackTheme(null), null));
-
-    test('invalid', () {
-      expectLater(
-        log.onRecord,
-        emits(severeLogOf(contains('Unable to read themes. See https://github.com/forus-labs/cauldron/tree/master/nitrogen#themes.'))),
-      );
-      expect(() => Configuration.parseFallbackTheme(loadYamlNode('true')), throwsA(isA<NitrogenException>()));
-    });
+    test('do not use package name & invalid name', () => expect(Configuration.parsePackage(loadYamlNode('true'), enabled: false), null));
   });
 
   group('parseFlutterAssets(...)', () {
diff --git a/nitrogen/test/src/configuration/key_test.dart b/nitrogen/test/src/configuration/key_test.dart
index f5d7149..3f68204 100644
--- a/nitrogen/test/src/configuration/key_test.dart
+++ b/nitrogen/test/src/configuration/key_test.dart
@@ -1,35 +1,31 @@
 import 'package:build/build.dart';
 import 'package:build_test/build_test.dart';
-import 'package:nitrogen/src/nitrogen_exception.dart';
-import 'package:nitrogen/src/configuration/key.dart';
 import 'package:test/test.dart';
 import 'package:yaml/yaml.dart';
 
+import 'package:nitrogen/src/configuration/key.dart';
+import 'package:nitrogen/src/nitrogen_exception.dart';
+
 // ignore_for_file: avoid_dynamic_calls
 
 void main() {
   group('parse', () {
     test('grpc-enum', () {
-      final key = Key.parse(loadYaml('key: grpc-enum').nodes['key']);
+      final key = Key.parse(loadYaml('key: grpc-enum')['key']);
       expect(key(['path', 'to', 'file.png']), 'TO_FILE');
     });
 
     test('file-name', () {
-      final key = Key.parse(loadYaml('key: file-name').nodes['key']);
-      expect(key(['path', 'to', 'file.png']), 'file');
-    });
-
-    test('null', () {
-      final key = Key.parse(null);
+      final key = Key.parse(loadYaml('key: file-name')['key']);
       expect(key(['path', 'to', 'file.png']), 'file');
     });
 
     test('invalid', () {
       expectLater(
         log.onRecord,
-        emits(severeLogOf(contains('Unable to read asset key. See https://github.com/forus-labs/cauldron/tree/master/nitrogen#key.'))),
+        emits(severeLogOf(contains('Unknown asset key: "invalid". See https://github.com/forus-labs/cauldron/tree/master/nitrogen#key.'))),
       );
-      expect(() => Key.parse(loadYaml('key: invalid').nodes['key']), throwsA(isA<NitrogenException>()));
+      expect(() => Key.parse(loadYaml('key: invalid')['key']), throwsA(isA<NitrogenException>()));
     });
   });
 }
diff --git a/nitrogen/test/src/file_system_test.dart b/nitrogen/test/src/file_system_test.dart
index 7a1b1d5..cc5aeb6 100644
--- a/nitrogen/test/src/file_system_test.dart
+++ b/nitrogen/test/src/file_system_test.dart
@@ -1,7 +1,8 @@
-import 'package:nitrogen/src/file_system.dart';
 import 'package:nitrogen_types/nitrogen_types.dart';
 import 'package:test/test.dart';
 
+import 'package:nitrogen/src/file_system.dart';
+
 void main() {
   group('AssetDirectory', () {
     test('empty path', () {
diff --git a/nitrogen/test/src/generators/asset_generator_test.dart b/nitrogen/test/src/generators/asset_generator_test.dart
index f66f75c..042a352 100644
--- a/nitrogen/test/src/generators/asset_generator_test.dart
+++ b/nitrogen/test/src/generators/asset_generator_test.dart
@@ -1,10 +1,11 @@
 import 'package:code_builder/code_builder.dart';
 import 'package:dart_style/dart_style.dart';
-import 'package:nitrogen/src/file_system.dart';
-import 'package:nitrogen/src/generators/asset_generator.dart';
 import 'package:nitrogen_types/nitrogen_types.dart';
 import 'package:test/test.dart';
 
+import 'package:nitrogen/src/file_system.dart';
+import 'package:nitrogen/src/generators/asset_generator.dart';
+
 const _classes = r'''
 import 'package:nitrogen_types/nitrogen_types.dart';
 
diff --git a/nitrogen/test/src/generators/theme_generator_test.dart b/nitrogen/test/src/generators/theme_generator_test.dart
index d605e29..5f221f7 100644
--- a/nitrogen/test/src/generators/theme_generator_test.dart
+++ b/nitrogen/test/src/generators/theme_generator_test.dart
@@ -1,9 +1,10 @@
 import 'package:code_builder/code_builder.dart';
-import 'package:nitrogen/src/file_system.dart';
-import 'package:nitrogen/src/generators/theme_generator.dart';
 import 'package:nitrogen_types/nitrogen_types.dart';
 import 'package:test/test.dart';
 
+import 'package:nitrogen/src/file_system.dart';
+import 'package:nitrogen/src/generators/theme_generator.dart';
+
 const _classes = r'''
 import 'package:nitrogen_types/nitrogen_types.dart';
 
diff --git a/nitrogen/test/src/libraries_test.dart b/nitrogen/test/src/libraries_test.dart
index 195830c..8575b5d 100644
--- a/nitrogen/test/src/libraries_test.dart
+++ b/nitrogen/test/src/libraries_test.dart
@@ -1,7 +1,8 @@
 import 'package:code_builder/code_builder.dart';
-import 'package:nitrogen/src/libraries.dart';
 import 'package:test/test.dart';
 
+import 'package:nitrogen/src/libraries.dart';
+
 void main() {
   test('format()', () {
     final library = Library((b) => b.body.addAll([
diff --git a/nitrogen/test/src/lints/reserved_keyword_lint_test.dart b/nitrogen/test/src/lints/reserved_keyword_lint_test.dart
index 4714c58..f853963 100644
--- a/nitrogen/test/src/lints/reserved_keyword_lint_test.dart
+++ b/nitrogen/test/src/lints/reserved_keyword_lint_test.dart
@@ -1,10 +1,11 @@
 import 'package:build/build.dart';
 import 'package:build_test/build_test.dart';
+import 'package:nitrogen_types/nitrogen_types.dart';
+import 'package:test/test.dart';
+
 import 'package:nitrogen/src/file_system.dart';
 import 'package:nitrogen/src/lints/reserved_keyword_lint.dart';
 import 'package:nitrogen/src/nitrogen_exception.dart';
-import 'package:nitrogen_types/nitrogen_types.dart';
-import 'package:test/test.dart';
 
 void main() {
   group('lintReservedKeyword', () {
diff --git a/nitrogen/test/src/walker_test.dart b/nitrogen/test/src/walker_test.dart
index 85518a2..be491d3 100644
--- a/nitrogen/test/src/walker_test.dart
+++ b/nitrogen/test/src/walker_test.dart
@@ -1,9 +1,10 @@
 import 'package:build/build.dart';
-import 'package:nitrogen/src/file_system.dart';
-import 'package:nitrogen/src/walker.dart';
 import 'package:nitrogen_types/nitrogen_types.dart';
 import 'package:test/test.dart';
 
+import 'package:nitrogen/src/file_system.dart';
+import 'package:nitrogen/src/walker.dart';
+
 void main() {
   group('walk(...)', () {
     test('ancestor asset directory is allowed', () {