WindowsAppSDK1.7以降で追加された、タイトルバーのTheme変更を試す

public sealed partial class MainWindow : Window
{
    public MainWindow()
    {
        this.InitializeComponent();
    }

    private void darkButton_Click(object sender, RoutedEventArgs e)
    {
        if (Content is FrameworkElement rootElement)
        {
            rootElement.RequestedTheme = ElementTheme.Dark;
            AppWindow.TitleBar.PreferredTheme = TitleBarTheme.Dark;
        }
    }

    private void lightButton_Click(object sender, RoutedEventArgs e)
    {
        if (Content is FrameworkElement rootElement)
        {
            rootElement.RequestedTheme = ElementTheme.Light;
            AppWindow.TitleBar.PreferredTheme = TitleBarTheme.Light;
        }
    }
}

<Grid Background="{ThemeResource ApplicationPageBackgroundThemeBrush}">
    <StackPanel
        HorizontalAlignment="Center"
        VerticalAlignment="Center"
        Orientation="Horizontal">
        <Button x:Name="darkButton" Click="darkButton_Click">Dark</Button>
        <Button x:Name="lightButton" Click="lightButton_Click">Light</Button>
    </StackPanel>
</Grid>