ux: content background in flyout

Signed-off-by: leo <longshuang@msn.cn>

Author: love-linger

src/Resources/Styles.axaml

@@ -215,7 +215,7 @@
           <Border Name="LayoutRoot"
                   Margin="4"
                   Padding="12"
-                  Background="{DynamicResource Brush.Popup}"
+                  Background="Transparent"
                   BorderThickness="0"
                   CornerRadius="{TemplateBinding CornerRadius}">
             <ScrollViewer HorizontalScrollBarVisibility="{TemplateBinding ScrollViewer.HorizontalScrollBarVisibility}"