I am not sure how you create your custom theme, but in a common scenario you should be able to set it like so:
And you will have to add the ThemeContent to your user control resources. I hope that information helps.
the Telerik team
Registration for Q1 2011 What’s New Webinar Week is now open. Mark your calendar for the week starting March 21st and book your seat
for a walk through all the exciting stuff we ship with the new release!