Fix hardcoded Avalonia update window bg

Author: Deadpikle

src/NetSparkle.UI.Avalonia/UpdateAvailableWindow.xaml

@@ -12,7 +12,7 @@
                  Width="700"
                  Height="600"
                  WindowStartupLocation="CenterScreen">
-  <Grid Background="#EEEEEE" ColumnDefinitions="Auto, *" Name="MainGrid"
+  <Grid ColumnDefinitions="Auto, *" Name="MainGrid"
         RowDefinitions="Auto, Auto, *, Auto">
     <Image Grid.Row="0"
            Grid.RowSpan="3"