SplitButton: Fix an issue with the Background property (patch #10883).

ExtendedWPFToolkitSolution/Src/WPFToolkit.Extended/SplitButton/Themes/Generic.xaml

@@ -31,12 +31,13 @@
         <Setter Property="IsTabStop" Value="False" />
         <Setter Property="HorizontalContentAlignment" Value="Center" />
         <Setter Property="VerticalContentAlignment" Value="Center" />
+        <Setter Property="Background" Value="{DynamicResource {x:Static SystemColors.ControlBrushKey}}" />
         <Setter Property="Padding" Value="3"/>
         <Setter Property="Template">
             <Setter.Value>
                 <ControlTemplate TargetType="{x:Type local:SplitButton}">
                     <Grid x:Name="MainGrid" SnapsToDevicePixels="True">
-                        <chrome:ButtonChrome x:Name="ControlChrome" RenderEnabled="{TemplateBinding IsEnabled}" CornerRadius="2.75">
+                        <chrome:ButtonChrome x:Name="ControlChrome" Background="{TemplateBinding Background}" RenderEnabled="{TemplateBinding IsEnabled}" CornerRadius="2.75">
                             <Grid>
                                 <Grid.ColumnDefinitions>
                                     <ColumnDefinition Width="*" />