Update dialog theme in ScratchPad page to match current app theme (#2028)

Author: Zakariathr22

WinUIGallery/Samples/ControlPages/Fundamentals/ScratchPadPage.xaml.cs

@@ -118,6 +118,7 @@ private async void ResetToDefaultClick(object sender, RoutedEventArgs e)
         dialog.PrimaryButtonText = "Reset";
         dialog.CloseButtonText = "Cancel";
         dialog.DefaultButton = ContentDialogButton.Primary;
+        dialog.RequestedTheme = this.ActualTheme;
 
         var result = await dialog.ShowAsync();
         if (result == ContentDialogResult.Primary)