http://www.jamesward.com/2010/07/30/how-to-define-styles-on-skins-in-flex-4/
<?xml version="1.0"?>
<s:SparkSkin xmlns:fx="http://ns.adobe.com/mxml/2009"
xmlns:s="library://ns.adobe.com/flex/spark">
<fx:Script>
import mx.utils.ColorUtil;
</fx:Script>
<s:states>
<s:State name="up"/>
<s:State name="over"/>
<s:State name="down"/>
</s:states>
<s:Rect width="100%" height="100%">
<s:fill>
<s:SolidColor color="{getStyle('backgroundColor')}"
color.over="{ColorUtil.adjustBrightness2(getStyle('backgroundColor'), 64)}"
color.down="{ColorUtil.adjustBrightness2(getStyle('backgroundColor'), -32)}"
/>
</s:fill>
</s:Rect>
<s:Label id="labelDisplay" paddingBottom="3" paddingLeft="3" paddingRight="3" paddingTop="6"/>
</s:SparkSkin>