The theming mechanism in RadControls for Silverlight does not
support the Silverlight 3
runtime - this is valid for both the current release of the controls, as well as all previous releases. We are planning to include theming support for Silverlight 3 in our Q3 2009 release, which is due around October.
In case you are interested in applying your custom theme to a Silverlight 2
application, let me know and I will provide an example.
All the best,
the Telerik team