Flex中如何改变FormItem容器的指示器（indicator）皮肤的例子

<?xml version="1.0" encoding="utf-8"?> 
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml" 
        layout="vertical" 
        verticalAlign="top" 
        backgroundColor="white"> 
  
    <mx:Style> 
        FormItem { 
            indicatorSkin: Embed(source="assets/asterisk_yellow.png"); 
            indicatorGap: 24; /* pixels */ 
        } 
  
        FormItemLabel { 
            textAlign: left; 
        } 
    </mx:Style> 
  
    <mx:Form> 
        <mx:FormHeading label="FORM HEADING" /> 
        <mx:FormItem label="Name:" required="true"> 
            <mx:TextInput /> 
        </mx:FormItem> 
        <mx:FormItem label="Email:" required="true"> 
            <mx:TextInput /> 
        </mx:FormItem> 
        <mx:FormItem label="Phone number:"> 
            <mx:TextInput /> 
        </mx:FormItem> 
    </mx:Form> 
  
</mx:Application>