App.xaml

<Application x:Class="OpenMediaDownloader.App"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:local="clr-namespace:OpenMediaDownloader"
             StartupUri="Windows/MainWindow.xaml">
    <Application.Resources>
        <!--https://stackoverflow.com/questions/3173364/white-border-around-groupbox-->
        <ResourceDictionary>
            <BorderGapMaskConverter x:Key="BorderGapMaskConverter"/>
            <Style TargetType="{x:Type GroupBox}">
                <Setter Property="BorderBrush" Value="#D5DFE5" />
                <Setter Property="BorderThickness" Value="1" />
                <Setter Property="Template">
                    <Setter.Value>
                        <ControlTemplate TargetType="{x:Type GroupBox}">
                            <Grid SnapsToDevicePixels="true">
                                <Grid.ColumnDefinitions>
                                    <ColumnDefinition Width="6" />
                                    <ColumnDefinition Width="Auto" />
                                    <ColumnDefinition Width="*" />
                                    <ColumnDefinition Width="6" />
                                </Grid.ColumnDefinitions>
                                <Grid.RowDefinitions>
                                    <RowDefinition Height="Auto" />
                                    <RowDefinition Height="Auto" />
                                    <RowDefinition Height="*" />
                                    <RowDefinition Height="6" />
                                </Grid.RowDefinitions>
                                <Border CornerRadius="4" Grid.Row="1" Grid.RowSpan="3" Grid.Column="0" Grid.ColumnSpan="4" BorderThickness="{TemplateBinding BorderThickness}" BorderBrush="Transparent" Background="{TemplateBinding Background}" />
                                <Border Name="Header" Padding="3,1,3,0" Grid.Row="0" Grid.RowSpan="2" Grid.Column="1">
                                    <ContentPresenter ContentSource="Header" RecognizesAccessKey="true" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" />
                                </Border>
                                <ContentPresenter Grid.Row="2" Grid.Column="1" Grid.ColumnSpan="2" Margin="{TemplateBinding Padding}" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" />
                                <Border Grid.Row="1" Grid.RowSpan="3" Grid.ColumnSpan="4"  BorderThickness="{TemplateBinding BorderThickness}" BorderBrush="{TemplateBinding BorderBrush}" CornerRadius="3">
                                    <Border.OpacityMask>
                                        <MultiBinding Converter="{StaticResource BorderGapMaskConverter}" ConverterParameter="7">
                                            <Binding ElementName="Header" Path="ActualWidth" />
                                            <Binding Path="ActualWidth" RelativeSource="{RelativeSource Self}" />
                                            <Binding Path="ActualHeight" RelativeSource="{RelativeSource Self}" />
                                        </MultiBinding>
                                    </Border.OpacityMask>
                                </Border>
                            </Grid>
                        </ControlTemplate>
                    </Setter.Value>
                </Setter>
            </Style>
        </ResourceDictionary>
    </Application.Resources>
</Application>