Flex中如何利用<mx:style>改变ToolTip外观的例子

<?xml version="1.0" encoding="utf-8"?> 
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml" 
        layout="vertical" 
        verticalAlign="middle" 
        backgroundColor="white"> 
  
    <mx:Style> 
        ToolTip { 
            backgroundColor: black; 
            backgroundAlpha: 1.0; 
            cornerRadius: 0; 
            color: white; 
        } 
    </mx:Style> 
  
    <mx:Button label="Roll over to see tool tip" 
            toolTip="The quick brown fox jumped over the lazy dog." /> 
  
</mx:Application>