Rolf,
No, there's no way to modify the looks of the Fan Box. Years ago, Facebook allowed the inclusion of your own CSS file where you could override the looks however you wanted. That was removed due to rampant abuse, and now, there's no ways to alter the looks.
Sorry for the trouble. If you find some site doing it differently, let us know, but the above has been true for quite a while.
Thanks,
Alex