I just spent most of the day trying to figure out a problem with a custom item renderer skin, so I thought I'd write it down here in case anyone else comes across this.
Basically I have a Tree control that contains a list of items that are rendered by my custom item renderer. Each renderer has a background gradient skin applied by the borderSkin style. Unfortunately there's a line in the List class that really screws this up:
renderer.styleName = this;
This is called when the Tree (or List) class creates a new item renderer and effectively replaces any style you assign to your item renderer with that of the Tree/List component. Therefore any style you assign to your Tree/List automatically gets inherited by your item renderer. I can't for the life of me think of one situation where anybody would ever want to do this!!
The simple fix is to re-apply the styleName in your item renderer, for example on your creationComplete even:
<mx:Canvas creationComplete="styleName='MyItemRendererStyle'" />
There's a Flex bug entry here: http://bugs.adobe.com/jira/browse/SDK-15002
Please vote for the bug so it gets fixed soon!!!