Bryant Likes wrote a good post about choosing how to add a functionality to an existing control : behaviour, subclassing or attached property along with an example for each.

Behaviors vs Subclassing in Silverlight

Timmy Kokke wrote a detailed post about how to style a treeview.

http://geekswithblogs.net/tkokke/archive/2009/06/29/styling-a-treeview-in-silverlight-and-blend-3.aspx