Use StyleManager.loadStyleDeclarations() to load CSS-based SWF file at run time
<?xml version="1.0"?>
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml">
<mx:Script>
import mx.styles.StyleManager;
public function applyRuntimeStyleSheet():void {
StyleManager.loadStyleDeclarations("a.swf")
}
</mx:Script>
<mx:Label text="Click the button to load a new CSS-based SWF file" />
<mx:Button id="b1" label="Click Me" click="applyRuntimeStyleSheet()" />
</mx:Application>