Use themed color for CodeStyleKey/CodeBlockStyleKey

Author: jcansdale

src/GitHub.UI/Assets/Markdown.xaml

@@ -43,11 +43,13 @@
   </Style>
   <Style TargetType="{x:Type Paragraph}" x:Key="{x:Static markdig:Styles.CodeBlockStyleKey}">
     <Setter Property="Foreground" Value="{DynamicResource VsBrush.Text}" />
+    <Setter Property="Background" Value="{DynamicResource GitHubBranchNameBackgroundBrush}" />
     <Setter Property="Padding" Value="2" />
     <Setter Property="FontFamily" Value="Consolas, Lucida Sans Typewriter, Courier New" />
   </Style>
   <Style TargetType="{x:Type Run}" x:Key="{x:Static markdig:Styles.CodeStyleKey}">
     <Setter Property="Foreground" Value="{DynamicResource VsBrush.Text}" />
+    <Setter Property="Background" Value="{DynamicResource GitHubBranchNameBackgroundBrush}" />
     <Setter Property="FontFamily" Value="Consolas, Lucida Sans Typewriter, Courier New" />
   </Style>
 </ResourceDictionary>