Load styles from css file at run time

Hello :). I am trying to load different styles at run time. In my css files there is only description of the Application tag. If I point to the file in my mxml code:

<mx:Style source="name_of_the_file.css"/>

it is working. The problem is that I don’t know how to do this with as3. I have managed to load data with this:

StyleManager.loadStyleDeclarations( 'name_of_the_file.swf' );

My question is how I can load not swf but .css file like in the example above. Thank you in advance. It must be something simple but now I can’t figure it out.