<?xml version="1.0" encoding="utf-8"?>
<!-- http://blog.flexexamples.com/2008/02/29/styling-alert-controls-in-flex-using-the-stylemanager-class-and-setstyle-methods/ -->
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml"
        layout="vertical"
        verticalAlign="middle"
        backgroundColor="white"
        creationComplete="init();">

<mx:Script>
        <![CDATA[
            import mx.controls.Alert;
            import mx.styles.StyleManager;

private var alert:Alert;
            private var alertCSS:CSSStyleDeclaration;

private function init():void {
                alertCSS = StyleManager.getStyleDeclaration("Alert");
            }

private function showAlert(color:Object):void {
                alertCSS.setStyle("modalTransparencyColor", color);
                alertCSS.setStyle("themeColor", color);
                alert = Alert.show("The quick brown fox");
            }
        ]]>
    </mx:Script>

<mx:ApplicationControlBar dock="true">
        <mx:Button label="Red"
                themeColor="red"
                click="showAlert('red');" />
        <mx:Button label="Orange"
                themeColor="haloOrange"
                click="showAlert('haloOrange');" />
        <mx:Button label="Yellow"
                themeColor="yellow"
                click="showAlert('yellow');" />
        <mx:Button label="Green"
                themeColor="haloGreen"
                click="showAlert('haloGreen');" />
        <mx:Button label="Blue"
                themeColor="haloBlue"
                click="showAlert('haloBlue');" />
    </mx:ApplicationControlBar>

</mx:Application>