The following example shows how you can style a Flex Alert control using the static StyleManager.getStyleDeclaration() method, and the setStyle() method
<?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 foxStyling Alert controls in Flex using the StyleManager class and setStyle() methods_CSSStyleDeclaration");
            }
        
]]>
    
</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>