Cannot set the Background color of a TitleBar

[brunosonnino]

Describe the bug

You cannot set the background color of a TitleBar by setting its Background property or setting AppWindow.TitleBar.BackgroundColor

Steps to reproduce the bug

• Create a Blank WinUI3 app
• Upgrade the WindowsAppSdk Nuget package to 1.6 experimental 2
• Change the project file to add

<WindowsSdkPackageVersion>10.0.19041.35-preview</WindowsSdkPackageVersion>

Add this code in MainWindow.xaml.cs

public MainWindow()
{
    this.InitializeComponent();
    ExtendsContentIntoTitleBar = true;
    AppWindow.TitleBar.BackgroundColor = Colors.Red;
}

Add this code to MainWindow.xaml:

<Grid>
    <Grid.RowDefinitions>
        <RowDefinition Height="Auto" />
        <RowDefinition Height="*" />
    </Grid.RowDefinitions>
    <TitleBar Title="Title" Subtitle="SubTitle" IsBackButtonVisible="True"
            IsPaneToggleButtonVisible="True" Background="Red">
        <TitleBar.IconSource>
            <FontIconSource Glyph="&#xe713;" />
        </TitleBar.IconSource>
    </TitleBar>
    <Grid Grid.Row="1" Background="Aqua">
    </Grid>
</Grid>

Run the project. Although the TitleBar background is set in two places, the background is still transparent

The workaround to this to add a control below the title bar with the background

This project has a sample of the bug https://github.com/brunosonnino/TitleBarBugs/tree/main/3%20-%20TitleBarBackground

Expected behavior

The title bar should change colors

Screenshots

NuGet package version

WinUI 3 - Windows App SDK 1.6 Experimental 2: 1.6.240701003-experimental2

Windows version

Windows 11 (22H2): Build 22621

Additional context

No response

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