I am very interested in this issue because I will add this feature to my diagram modeling tool.
Search Sample using Silverlight.FX
Link: Themes for Silverlight Applications http://www.nikhilk.net/Silverlight-Themes.aspx
This look very nice but I don’t want to use Silverlight.FX framework just for this simple theme. Instead I recommend you to have a look of this lighter version.
Link: A New Set of Control Skins http://blogs.msdn.com/b/corrinab/archive/2008/03/24/a-new-control-skin-set.aspx