Support themes!

Author: donokuda

src/GitHub.VisualStudio/SharedDictionary.xaml

@@ -50,7 +50,7 @@
   </Style>
 
   <Style x:Key="VerticalSeparator" TargetType="{x:Type Separator}">
-    <Setter Property="Background" Value="#FFCFD6E6" />
+    <Setter Property="Background" Value="{DynamicResource GitHubHeaderSeparatorBrush}" />
     <Setter Property="Margin" Value="3,0,3,0" />
     <Setter Property="LayoutTransform">
       <Setter.Value>