When color theme is used, ContentDialog covers the whole page

[under3415]

Steps to replicate:

1. Create new UWP app
2. Grab a color scheme from Fluent XAML Theme Editor and add it to your project
3. In App.Xaml add this:

     <Application.Resources>
      <ResourceDictionary>
       <ResourceDictionary.MergedDictionaries>
           <ResourceDictionary Source="Dictionary1.xaml"/>
      </ResourceDictionary.MergedDictionaries>
     </ResourceDictionary>
    </Application.Resources>

4. Add a TextBlock and Loaded event to MainPage
5. Add ContentDialog call to Loaded event

   private async void Page_Loaded(object sender, Windows.UI.Xaml.RoutedEventArgs e)
        {
            ContentDialog noWifiDialog = new ContentDialog
            {
                Title = "No wifi connection",
                Content = "Check your connection and try again.",
                CloseButtonText = "Ok"
            };

            _ = await noWifiDialog.ShowAsync();
        }

6. Run the app and see how ContentDialog covers up the TextBlock. If you remove the color theme this does not happen.

[under3415]

You need to add this to your theme to fix the ContentDialog issue

      <SolidColorBrush x:Key="SystemControlPageBackgroundMediumAltMediumBrush" Color="#99000000" />
                            <StaticResource x:Key="ContentDialogLightDismissOverlayBackground" ResourceKey="SystemControlPageBackgroundMediumAltMediumBrush" />