fix(ResourceDictionary): Default theme may not be valid on first call

There are no tests for this specific change as it impacts the Application.Current. The validation for this change can be done through the Uno Gallery at unoplatform/Uno.Gallery@9fbf149. The background should be dark when system settings are dark.

src/Uno.UI/UI/Xaml/Application.cs

@@ -46,6 +46,7 @@ public partial class Application
 		private bool _themeSetExplicitly = false;
 		private ApplicationTheme? _requestedTheme;
 		private bool _systemThemeChangesObserved = false;
+		private string _requestedThemeForResources;
 
 		static Application()
 		{
@@ -81,11 +82,7 @@ public ApplicationTheme RequestedTheme
 		{
 			get
 			{
-				if (InternalRequestedTheme == null)
-				{
-					// just cache the theme, but do not notify about a change unnecessarily	
-					InternalRequestedTheme = GetDefaultSystemTheme();
-				}
+				EnsureInternalRequestedTheme();
 				return InternalRequestedTheme.Value;
 			}
 			set
@@ -98,6 +95,15 @@ public ApplicationTheme RequestedTheme
 			}
 		}
 
+		private void EnsureInternalRequestedTheme()
+		{
+			if (InternalRequestedTheme == null)
+			{
+				// just cache the theme, but do not notify about a change unnecessarily	
+				InternalRequestedTheme = GetDefaultSystemTheme();
+			}
+		}
+
 		private ApplicationTheme? InternalRequestedTheme
 		{
 			get => _requestedTheme;
@@ -120,7 +126,16 @@ internal static void UpdateRequestedThemesForResources()
 				};
 		}
 
-		internal string RequestedThemeForResources { get; private set; }
+		internal string RequestedThemeForResources
+		{
+			get
+			{
+				EnsureInternalRequestedTheme();
+				return _requestedThemeForResources;
+			}
+
+			private set => _requestedThemeForResources = value;
+		}
 
 		internal ElementTheme ActualElementTheme => (_themeSetExplicitly, RequestedTheme) switch
 		{