更新 Overlay 背景为透明

将 `Overlay` 网格的 `Background` 属性从 `{ThemeResource TabViewItemHeaderBackground}` 更改为 `Transparent`，以使其背景透明。

Author: qian-o

src/WinUI.Dock/Themes/Styles.xaml

@@ -708,7 +708,7 @@
                               IsHitTestVisible="False">
                             <Grid x:Name="Overlay"
                                   Margin="0,0,0,4"
-                                  Background="{ThemeResource TabViewItemHeaderBackground}"
+                                  Background="Transparent"
                                   BorderBrush="{TemplateBinding BorderBrush}"
                                   BorderThickness="{ThemeResource TabViewSelectedItemBorderThickness}"
                                   CornerRadius="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=CornerRadius, Converter={StaticResource TopCornerRadiusFilterConverter}}"