Thanks for your question.
Unfortunately, there no easy way around this problem at this time. We will definitely think of either styling all the default controls with our themes or vice versa - providing an equal theme as one of the available themes for the default Silverlight Toolkit controls. We realize that this will be really beneficial for users like you, and we will surely work in this direction for the next Q2 release. I am afraid that we cannot promise to deliver such a theme with Q2 as we already started working on other major tasks. Nevertheless, if we do not manage to include such a theme in the Q2 release, we will be working on this task right after that.
the Telerik team