Set window Foreground property to enable theming

src/Avalonia.Themes.Default/Window.xaml

@@ -1,5 +1,6 @@
 <Style xmlns="https://github.com/avaloniaui" Selector="Window">
   <Setter Property="Background" Value="{DynamicResource ThemeBackgroundBrush}"/>
+  <Setter Property="Foreground" Value="{DynamicResource ThemeForegroundBrush}"/>
   <Setter Property="FontSize" Value="{DynamicResource FontSizeNormal}"/>
   <Setter Property="Template">
     <ControlTemplate>
@@ -15,4 +16,4 @@
       </Border>
     </ControlTemplate>
   </Setter>
-</Style>
+</Style>