<?xml version="1.0" encoding="utf-8"?>
<!-- http://blog.flexexamples.com/2007/09/18/setting-a-flex-applications-style-name/ -->
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml"
        layout="vertical"
        verticalAlign="middle">

    <mx:Script>
        <![CDATA[
            private function plainStyle():void {
                Application.application.setStyle("styleName", "plain");
            }

            private function defaultStyle():void {
                Application.application.setStyle("styleName", undefined);
            }
        ]]>
    </mx:Script>

    <mx:ApplicationControlBar dock="true">
        <mx:Button label="styleName = 'plain'"
                click="plainStyle();" />
        <mx:Button label="styleName = undefined"
                click="defaultStyle();" />
    </mx:ApplicationControlBar>

    <mx:Button label="Button 1" />
    <mx:Button label="Button 2" />
    <mx:Button label="Button 3" />

</mx:Application>