Hello Tobias Riley,
Thank you for your interest in RadSpellChecker. Currntly we do not support theming the pop-ups. Actually, we are going to introduce this feature in our service pack, which is to be released in two weeks - around April 13. When that happens you will be able to theme the pop-ups this way:
Of course if you want to use a specific theme you should add the appropriate assembly.
We are sorry if this caused any inconvenience. Contact us again if you have other questions.
the Telerik team