Fix dark theme foreground color.

Author: grokys

src/GitHub.VisualStudio/Views/GitHubPane/PullRequestFilesView.xaml

@@ -46,6 +46,7 @@
         <TreeView.ItemContainerStyle>
             <Style TargetType="TreeViewItem">
                 <Setter Property="IsExpanded" Value="True"/>
+                <Setter Property="Foreground" Value="{DynamicResource GitHubVsWindowText}"/>
             </Style>
         </TreeView.ItemContainerStyle>
         <TreeView.Resources>