Update separator colors

Author: donokuda

src/GitHub.VisualStudio.UI/Styles/ThemeDark.xaml

@@ -12,8 +12,8 @@
 
   <Color x:Key="GitHubActionLinkItemColor">#FF0097FB</Color>
   <SolidColorBrush x:Key="GitHubActionLinkItemBrush" Color="{StaticResource GitHubActionLinkItemColor}" PresentationOptions:Freeze="true" />
-  
-  <Color x:Key="GitHubHeaderSeparatorColor">#FF2D2D30</Color>
+
+  <Color x:Key="GitHubHeaderSeparatorColor">#FF3f3f46</Color>
   <SolidColorBrush x:Key="GitHubHeaderSeparatorBrush" Color="{StaticResource GitHubHeaderSeparatorColor}" PresentationOptions:Freeze="true" />
 
   <Color x:Key="GitHubPaneTitleColor">#FFFFFFFF</Color>

src/GitHub.VisualStudio.UI/Styles/ThemeLight.xaml

@@ -13,7 +13,7 @@
   <Color x:Key="GitHubActionLinkItemColor">#FF0E70C0</Color>
   <SolidColorBrush x:Key="GitHubActionLinkItemBrush" Color="{StaticResource GitHubActionLinkItemColor}" PresentationOptions:Freeze="true" />
 
-  <Color x:Key="GitHubHeaderSeparatorColor">#FFEEEEF2</Color>
+  <Color x:Key="GitHubHeaderSeparatorColor">#FFcccedb</Color>
   <SolidColorBrush x:Key="GitHubHeaderSeparatorBrush" Color="{StaticResource GitHubHeaderSeparatorColor}" PresentationOptions:Freeze="true" />
 
   <Color x:Key="GitHubPaneTitleColor">#FF1B293E</Color>

src/GitHub.VisualStudio.UI/Views/Documents/PullRequestPageView.xaml

@@ -81,9 +81,9 @@
 
         <Rectangle DockPanel.Dock="Top" Style="{StaticResource Separator}" Height="2" Margin="0 16 0 8"/>
 
-        <Border BorderBrush="#D1D5DA" BorderThickness="1" Margin="0 4">
+        <Border BorderBrush="{DynamicResource GitHubHeaderSeparatorBrush}" BorderThickness="1" Margin="0 4">
             <StackPanel Orientation="Vertical">
-                <Border Padding="8" Background="#F6F8FA" BorderBrush="#D1D5DA" BorderThickness="0 0 0 1">
+                <Border Padding="8" Background="{DynamicResource GitHubBranchNameBackgroundBrush}" BorderBrush="{DynamicResource GitHubHeaderSeparatorBrush}" BorderThickness="0 0 0 1">
                     <StackPanel Orientation="Horizontal" DockPanel.Dock="Left" >
                         <v:ActorAvatarView Width="16" 
                                                Height="16"