H.W.
How about getting them excited about a new joint theme...such as Dr. Seuss, ABC's, some other movie they both enjoy, safari animals, etc? Sounds like they like character themes, and you didn't mention their ages, but be careful about taking this away from them if they may find solid colors or stripes boring. You want them to be excited about their new room (not that I am saying they can't be excited about some solid colors/designs/whatever, but you may have to work at it!) Good luck!