Adding dedicated brush key for content of InRibbonGallery

Author: batzen

Fluent.Ribbon/Themes/Controls/InRibbonGallery.xaml

@@ -449,7 +449,7 @@
                     BorderBrush="{DynamicResource Fluent.Ribbon.Brushes.Control.Border}"
                     BorderThickness="1 1 1 1"
                     ClipToBounds="True">
-                <Grid Background="{DynamicResource Fluent.Ribbon.Brushes.White}">
+                <Grid Background="{DynamicResource Fluent.Ribbon.Brushes.InRibbonGallery.Content.Background}">
                     <ScrollViewer x:Name="ScrollViewer"
                                   Style="{DynamicResource Fluent.Ribbon.Styles.ScrollViewer}"
                                   Fluent:ScrollViewerAttachedProperties.ScrollBarStyle="{DynamicResource Fluent.Ribbon.Styles.InRibbonGallery.ScrollBar}"

Fluent.Ribbon/Themes/Themes/Theme.Template.xaml

@@ -179,6 +179,9 @@
 
     <!-- Gallery -->
     <SolidColorBrush x:Key="Fluent.Ribbon.Brushes.Gallery.Header.Background" Color="{StaticResource Fluent.Ribbon.Colors.Gray3}" options:Freeze="True" />
+    
+    <!-- InRibbonGallery -->
+    <SolidColorBrush x:Key="Fluent.Ribbon.Brushes.InRibbonGallery.Content.Background" Color="{StaticResource Fluent.Ribbon.Colors.White}" options:Freeze="True" />
 
     <!-- GalleryGroupContainer -->
     <SolidColorBrush x:Key="Fluent.Ribbon.Brushes.GalleryGroupContainer.Header.Background" Color="{StaticResource Fluent.Ribbon.Colors.Gray8}" options:Freeze="True" />