[flexcoders] How do I reference the background Image of the application?

[luvfotography]

Hi, I want to change the backgroundAlpha of the Application's 
background image.  But how do I reference it? The application tag 
doesn't allow an id??

Here's what I have:
This blows up:
Application.application.SetStyle("backgroundAlpha", .3);

in the Styles I have:
Application { 
        color: #ffffff; 
        borderColor: #B7BABC; 
        backgroundColor: #000000;
        background-alpha: .5;
        background-image: Embed("/images/bg.png");         
        borderStyle: none; 
        borderThickness: 0; }

I want to be able to change the background alpha dynamically.
thanks,