Update demo toolbar background to standard Primary brush

Updates the custom background ToolBar in the demo app to use the standard Primary brush instead of the Dark variant. This continues to demonstrate that the overflow button correctly inherits the ToolBar's background color while using a more conventional theme resource.

Author: Keboo

src/MainDemo.Wpf/MenusAndToolBars.xaml

@@ -380,7 +380,7 @@
 
     <smtx:XamlDisplay Margin="0,16,0,16" UniqueKey="menus_toolbar_custom_bg">
       <ToolBarTray>
-        <ToolBar Background="{DynamicResource MaterialDesign.Brush.Primary.Dark}"
+        <ToolBar Background="{DynamicResource MaterialDesign.Brush.Primary}"
                  ClipToBounds="False"
                  Style="{StaticResource MaterialDesignToolBar}">
           <Button Content="{materialDesign:PackIcon Kind=ContentSave}" ToolTip="Save" />