flexstyle.mxml
<?xml version="1.0" encoding="utf-8"?>
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml" layout="absolute">
<!--http://bbs.5460.net/ss-xs/?uid-10793452-action-viewspace-itemid-635270-->
<mx:Style>
 .solidBorder{
  color:#FF0000;
  fontSize:18;
  fontWeight:bold;
 }
 Button{
  fontSize:18;
  color:#00FF00;
 }
</mx:Style>
<mx:Script source="setStyle.as"/>
<mx:Style source="style.css"/>
 <mx:Panel width="447" height="410" layout="absolute" horizontalCenter="0" verticalCenter="0">
  <mx:Label id="label1" styleName="solidBorder" text="使用本地样式定义styleName" x="78" y="24" />
  <mx:Button label="使用本地样式定义" x="78" y="88" width="185"/>
  <mx:Button label="使用内联样式" x="78" y="158" width="120" fontSize="14" color="#000000"/>
  <mx:Button label="使用外部样式表" x="78" y="231" width="185" styleName="buttonstyle" />
  <mx:Button id="myButton" label="setStyle设置" width="185" fontSize="20" x="78" y="308" color="#000000"  styleName="buttonstyle" initialize="initButton();" />
 </mx:Panel>
</mx:Application>

style.css

 .buttonstyle{
  color:#FF0000;
  fontSize:18;
  fontWeight:bold;
 }

setStyle.as

private function initButton():void {
 myButton.setStyle("paddingTop",12);
 myButton.setStyle("paddingBottom",12);
 //myButton.setStyle("width",10);
 myButton.setStyle("fontSize",10);
 mx.core.Application.application.myButton.label = '使用setStyle设置css'
}