<?xml version="1.0" encoding="utf-8"?>
<!-- http://blog.flexexamples.com/2008/05/26/setting-the-border-color-on-a-numericstepper-control-in-flex/ -->
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml"
        layout="vertical"
        verticalAlign="middle"
        backgroundColor="white">

    <mx:Style>
        NumericStepper {
            borderColor: red;
        }
    </mx:Style>

    <mx:NumericStepper id="numericStepper" />

</mx:Application>