[flexcoders] How to use embed metadata in css in flex4

[j2me_soul]

I try to use a bitmap be the background of the titlewindow conponent in flex4 ?
I write this, but it doesn't work. It still show the default spark style.
<fx:style>
@namespace s "library://ns.adobe.com/flex/spark";
@namespace mx "library://ns.adobe.com/flex/halo";
mx|TitleWindow
{
 background-image: Embed("assets/background/titlewindow.png");
 font-size: 24;
}
</fx:style>
 <mx:TitleWindow id="btnClickWindow" showCloseButton="true" 
styleName="firstrate" width="200" height="300" title="title">
  <s:Button label="Button"/>
 </mx:TitleWindow>