Enable full theme support for main view

samples/ControlCatalog/MainView.xaml

@@ -1,6 +1,9 @@
 <UserControl xmlns="https://github.com/avaloniaui"
         xmlns:pages="clr-namespace:ControlCatalog.Pages"
-        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
+        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+        Background="{DynamicResource ThemeBackgroundBrush}"
+        TextBlock.Foreground="{DynamicResource ThemeForegroundBrush}"
+        TextBlock.FontSize="{DynamicResource FontSizeNormal}">
   <TabControl Classes="sidebar" Name="Sidebar">
     <TabControl.PageTransition>
       <CrossFade Duration="0.25"/>