PowerToys/src/settings-ui/Microsoft.PowerToys.Settings.UI/Themes/SettingsExpanderStyles.xaml

<ResourceDictionary
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" 
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:controls="using:Microsoft.PowerToys.Settings.UI.Controls"
    xmlns:muxc="using:Microsoft.UI.Xaml.Controls">

    <!-- Thickness -->
    <Thickness x:Key="ExpanderContentPadding">0</Thickness>
    <Thickness x:Key="ExpanderSettingMargin">56, 12, 40, 12</Thickness>

    <!-- Styles -->
    <!-- Setting used in a Expander header -->
    <Style x:Key="ExpanderHeaderSettingStyle" TargetType="controls:Setting">
        <Setter Property="Background" Value="Transparent" />
        <Setter Property="BorderThickness" Value="0" />
        <Setter Property="BorderBrush" Value="Transparent" />
        <Setter Property="Padding" Value="0, 14, 0, 14" />
        <Setter Property="Margin" Value="0"/>
        <Setter Property="HorizontalAlignment" Value="Stretch" />
    </Style>

    <Thickness x:Key="ExpanderChevronMargin">0,0,8,0</Thickness>

    <!-- Setting used in a Expander header -->
    <Style x:Key="ExpanderContentSettingStyle" TargetType="controls:Setting">
        <Setter Property="Background" Value="Transparent" />
        <Setter Property="BorderThickness" Value="0,1,0,0" />
        <Setter Property="BorderBrush" Value="{ThemeResource CardBorderBrush}" />
        <Setter Property="CornerRadius" Value="0" />
        <Setter Property="Padding" Value="{StaticResource ExpanderSettingMargin}" />
        <Setter Property="HorizontalAlignment" Value="Stretch" />
    </Style>

    <!-- Setting expander style -->
    <Style x:Key="SettingsExpanderStyle" TargetType="muxc:Expander">
        <Setter Property="Background" Value="{ThemeResource CardBackgroundBrush}" />
        <Setter Property="BorderThickness" Value="{ThemeResource CardBorderThickness}" />
        <Setter Property="BorderBrush" Value="{ThemeResource CardBorderBrush}" />
        <Setter Property="HorizontalAlignment" Value="Stretch" />
        <Setter Property="HorizontalContentAlignment" Value="Stretch" />
    </Style>

</ResourceDictionary>