terminal/src/cascadia/TerminalApp/App.xaml

<!-- Copyright (c) Microsoft Corporation. All rights reserved. Licensed under
the MIT License. See LICENSE in the project root for license information. -->
<Toolkit:XamlApplication
    x:Class="TerminalApp.App"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="using:TerminalApp"
    xmlns:Toolkit="using:Microsoft.Toolkit.Win32.UI.XamlHost"
    xmlns:TA="using:TerminalApp"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    mc:Ignorable="d">
    <!-- If you want to prove this works, then add `RequestedTheme="Light"` to
    the properties on the XamlApplication -->
    <Toolkit:XamlApplication.Resources>

        <ResourceDictionary>
            <ResourceDictionary.MergedDictionaries>
                <!-- Include the MUX Controls resources -->
                <XamlControlsResources xmlns="using:Microsoft.UI.Xaml.Controls"/>
                <ResourceDictionary>

                    <!-- We're going to apply this style to the root Grid acting
                    as the tab row, because we need to be able to set its
                    `Background` property to "{ThemeResource
                    ApplicationPageBackgroundThemeBrush}" so it will be colored
                    appropriately for the theme, regardless of what we set the
                    RequestedTheme to -->
                    <Style x:Name="BackgroundGridThemeStyle" TargetType="Grid">
                        <Setter Property="Background" Value="{ThemeResource ApplicationPageBackgroundThemeBrush}" />
                    </Style>

                    <!-- We need to manually create the error text brush as a
                    theme-dependent brush. SystemControlErrorTextForegroundBrush
                    is unfortunately static. -->
                    <SolidColorBrush x:Name="ErrorTextBrush" Color="{ThemeResource SystemErrorTextColor}" />

                    <!-- Suppress all padding except left around the tabs. The TabView looks far better like this. -->
                    <Thickness x:Key="TabViewHeaderPadding">8,0,0,0</Thickness>

                    <ResourceDictionary.ThemeDictionaries>
                        <ResourceDictionary x:Key="Dark">
                            <!-- Define resources for Dark mode here -->
                            <SolidColorBrush x:Key="TabViewBackground" Color="#FF333333" />
                        </ResourceDictionary>

                        <ResourceDictionary x:Key="Light">
                            <!-- Define resources for Light mode here -->
                            <SolidColorBrush x:Key="TabViewBackground" Color="#FFCCCCCC" />
                        </ResourceDictionary>

                    </ResourceDictionary.ThemeDictionaries>

                </ResourceDictionary>
            </ResourceDictionary.MergedDictionaries>
        </ResourceDictionary>


    </Toolkit:XamlApplication.Resources>
</Toolkit:XamlApplication>