Fix icon background in PullRequestReviewAuthoringView.

See #1744 (comment) for information on why this needs to be done here and not in `PullRequestFilesView`.

Author: grokys

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

@@ -10,8 +10,10 @@
                                              xmlns:c="clr-namespace:GitHub.VisualStudio.UI.Controls;assembly=GitHub.VisualStudio.UI"
                                              xmlns:models="clr-namespace:GitHub.Models;assembly=GitHub.App"
                                              xmlns:sampleData="clr-namespace:GitHub.SampleData;assembly=GitHub.App"
+                                             xmlns:theming="clr-namespace:Microsoft.VisualStudio.PlatformUI;assembly=Microsoft.VisualStudio.Imaging"
                                              Background="{DynamicResource GitHubVsToolWindowBackground}"
                                              Foreground="{DynamicResource GitHubVsWindowText}"
+                                             theming:ImageThemingUtilities.ImageBackgroundColor="{Binding RelativeSource={RelativeSource Self}, Path=Background.Color}"
                                              mc:Ignorable="d" d:DesignWidth="356" d:DesignHeight="800">
     <d:DesignProperties.DataContext>
         <sampleData:PullRequestReviewAuthoringViewModelDesigner/>