The following example allows you to use Cascading Style Sheets (CSS) to control a Flex Alert control’s modalTransparencyBlur, modalTransparency, modalTransparencyColor, and modalTransparencyDuration styles.
Full code after the jump.
<?xml version="1.0" encoding="utf-8"?>
<!-- http://blog.flexexamples.com/2007/08/14/changing-a-modal-alert-controls-blur-transparency-and-transparency-color/ -->
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml"
layout="vertical"
verticalAlign="middle"
backgroundColor="white"
creationComplete="showAlert()">
<mx:Style>
Alert {
modalTransparencyBlur: 5;
modalTransparency: 0.6;
modalTransparencyColor: #FF0000;
modalTransparencyDuration: 1000;
}
</mx:Style>
<mx:Script>
<![CDATA[
import mx.controls.Alert;
private var a:Alert;
private function showAlert():void {
a = Alert.show("text", "title", Alert.OK);
}
]]>
</mx:Script>
<mx:Button label="Show Alert" click="showAlert()" />
</mx:Application>
View source is enabled in the following example.





Or change the class:
// Setup new mofdal window class for popupManager
protected function setupOverlay(type:Class):void{
var ipm:IPopUpManager =Singleton.getInstance(”mx.managers::IPopUpManager”) as PopUpManager;
var o:Object = ipm;
var p:QName = new QName(mx_internal, “modalWindowClass”);
o[p] = type;
}
Just make sure the type class reference extends UIComponent :)
I have been Doing quite a bit with the PopUpManager, and found it way too locked down, which is why I have to step into mx_internal namespace :(