package components {

	import skins.MyComponent;
	import spark.components.supportClasses.Component;

	public class MyComponent extends Component {
		
		public function MyComponent() {
			super();
			var t:* = getStyle('skinClass');
			if(t == undefined){
				setStyle("skinClass", skins.MyComponent);
			}
		}
	}
}

This is an example of how to define a default skin in a component written in ActionScript 3 that extends a spark component.

Setting a Default Skin

Flex

ActionScript 3