chore: update theme brush to use the same as all others

Author: DevTKSS

src/DevTKSS.Uno.Samples.MvuxGallery/Presentation/Views/SimpleListPage.xaml

@@ -6,7 +6,7 @@
     xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
     xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
     mc:Ignorable="d"
-    Background="{ThemeResource ApplicationPageBackgroundThemeBrush}">
+    Background="{ThemeResource BackgroundBrush}">
 
     <ListView ItemsSource="{Binding GalleryImagesWithResw}"
               ItemTemplate="{StaticResource GalleryImageOverlayTemplate}"