https://learn.microsoft.com/en-us/uwp/api/windows.ui.xaml.frameworkelement.actualtheme?view=winrt-28000
FrameworkElement.ActualTheme Property (Windows.UI.Xaml) - Windows apps | Microsoft Learn
Gets the UI theme that is currently used by the element, which might be different than the RequestedTheme.
propertywindowsuixamlapps