Merged PR 21249: ActualTheme will not be Default

Default is used for RequestedTheme, but is not returned by ActualTheme, per [sample code](https://microsoft.sharepoint.com/teams/OSG_DevX/Final%20Review%20Specs/Forms/AllItems.aspx?id=%2Fteams%2FOSG%5FDevX%2FFinal%20Review%20Specs%2FRS3%20API%20spec%20%2D%20FrameworkElement%2EActualTheme%20%2D%20API%20review%20copy%2Edocx&parentview=7).

Calling this out because I've seen code that [tries to recover from an ActualTheme of "Default" by trying to guess what theme is actually in effect](https://microsoft.visualstudio.com/OS/_git/OSClient/pullrequest/10121384).

Author: oldnewthing

windows.ui.xaml/frameworkelement_actualtheme.md

@@ -19,6 +19,8 @@ Gets the UI theme that is currently used by the element, which might be differen
 
 A value of the enumeration, for example **Light**.
 
+The value of this property will not be **Default**.
+
 ## -remarks
 
 ## -see-also