<?xml version="1.0" encoding="utf-8"?>
<!-- http://blog.flexexamples.com/2008/05/27/setting-a-border-on-specific-sides-of-a-numericstepper-control-in-flex/ -->
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml"
        layout="vertical"
        verticalAlign="middle"
        backgroundColor="white">

    <mx:Style>
        NumericStepper {
            borderStyle: solid;
            borderSides: top, bottom;
        }
    </mx:Style>

    <mx:ApplicationControlBar dock="true">
        <mx:Form styleName="plain">
            <mx:FormItem label="borderThickness:">
                <mx:HSlider id="slider"
                        minimum="0"
                        maximum="10"
                        value="4"
                        snapInterval="1"
                        tickInterval="1"
                        liveDragging="true" />
            </mx:FormItem>
        </mx:Form>
    </mx:ApplicationControlBar>

    <mx:NumericStepper id="numericStepper"
            borderThickness="{slider.value}" />

</mx:Application>