Sounds like someone should file a bug to let Nokia know - it's either a bug (theme issue) with the FM Radio app, or it's a Plankton issue but at least Nokia can say "not our problem" once notified and the Plankton developer can work on a fix...

I'm surprised this issue has arisen to be honest... applications should be compatible with themes by and large as painting the window is a system level service unless the FM Radio applet is doing something wacky (which it shouldn't be doing, i.e. it's a bug or badly written application).