Add ability to customise inverse button colours

CHANGELOG.md

@@ -55,6 +55,22 @@ These styles can be used in your own code by including the `govuk-focused-box` S
 
 This change was made in [pull request #3819: Add linked image focus style](https://github.com/alphagov/govuk-frontend/pull/3819).
 
+#### Customise inverse button colours
+
+For non-GOV.UK branded websites, you can now change the colours of inverse buttons — buttons intended to be used on dark backgrounds.
+
+To change the inverse button's background colour, set the `$govuk-inverse-button-background-colour` Sass variable.
+
+To change the inverse button's text colour, set the `$govuk-inverse-button-text-colour` Sass variable.
+
+```scss
+@import "node_modules/govuk-frontend/govuk/base";
+
+$govuk-inverse-button-background-colour: govuk-colour("yellow");
+$govuk-inverse-button-text-colour: govuk-colour("black");
+@import "node_modules/govuk-frontend/govuk/components/button/index";
+```
+
 ### Breaking changes
 
 You must make the following changes when you migrate to this release, or your service might break.

packages/govuk-frontend/src/govuk/components/button/_index.scss

@@ -16,6 +16,20 @@ $govuk-button-background-colour: govuk-colour("green") !default;
 
 $govuk-button-text-colour: govuk-colour("white") !default;
 
+/// Inverted button component background colour
+///
+/// @type Colour
+/// @access public
+
+$govuk-inverse-button-background-colour: govuk-colour("white") !default;
+
+/// Inverted button component text colour
+///
+/// @type Colour
+/// @access public
+
+$govuk-inverse-button-text-colour: $govuk-brand-colour !default;
+
 @include govuk-exports("govuk/component/button") {
   $govuk-button-colour: $govuk-button-background-colour;
   $govuk-button-text-colour: $govuk-button-text-colour;
@@ -35,8 +49,8 @@ $govuk-button-text-colour: govuk-colour("white") !default;
   $govuk-warning-button-shadow-colour: govuk-shade($govuk-warning-button-colour, 60%);
 
   // Inverse button variables
-  $govuk-inverse-button-colour: govuk-colour("white");
-  $govuk-inverse-button-text-colour: govuk-colour("blue");
+  $govuk-inverse-button-colour: $govuk-inverse-button-background-colour;
+  $govuk-inverse-button-text-colour: $govuk-inverse-button-text-colour;
   $govuk-inverse-button-hover-colour: govuk-tint($govuk-inverse-button-text-colour, 90%);
   $govuk-inverse-button-shadow-colour: govuk-shade($govuk-inverse-button-text-colour, 30%);