<?xml version="1.0" encoding="utf-8"?>
<!-- http://blog.flexexamples.com/2008/05/23/setting-the-background-color-and-background-alpha-for-a-numericstepper-control-in-flex/ -->
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml"
        layout="vertical"
        verticalAlign="middle"
        backgroundColor="white">

    <mx:Script>
        <![CDATA[
            private function init():void {
                numericStepper.setStyle("backgroundAlpha", 0.5);
                numericStepper.setStyle("backgroundColor", "red");
            }
        ]]>
    </mx:Script>

    <mx:NumericStepper id="numericStepper"
            creationComplete="init();" />

</mx:Application>