Your sample code is not in your main application.
<fx:Style>
@namespace s "library://ns.adobe.com/flex/spark";
@namespace mx "library://ns.adobe.com/flex/mx";
s|FormItem
{
skinClass:ClassReference("skins.CustomFormItemSkins");
}
you have to add this style on your main application. That is where you have the Application container.
I think it should be right....