Wino-Mail/Wino.Core.WinUI/AppThemes/Default.xaml

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

    <x:String x:Key="ThemeName">Default</x:String>

    <SolidColorBrush x:Key="WinoApplicationBackgroundColor">Transparent</SolidColorBrush>
    <SolidColorBrush x:Key="AppBarBackgroundColor">Transparent</SolidColorBrush>

    <!--  Mica Template  -->
    <ResourceDictionary.ThemeDictionaries>
        <ResourceDictionary x:Name="Light">
            <!--  Reading Page Date/Name Group Header Background  -->
            <SolidColorBrush x:Key="MailListHeaderBackgroundColor">#ecf0f1</SolidColorBrush>
        </ResourceDictionary>
        <ResourceDictionary x:Name="Dark">
            <SolidColorBrush x:Key="MailListHeaderBackgroundColor">#1f1f1f</SolidColorBrush>
        </ResourceDictionary>
    </ResourceDictionary.ThemeDictionaries>
</ResourceDictionary>