It is possible, I believe. However before I continue I should let you know that Omni (the owner of another huge forum) plug ins on his MyBB Central site are all free, so you should go grab the Tabbed_Menu plug in as it's better and more functional.
Either way, both plug ins I assume run the same and they have a CSS page, you do all your designing with how they look and what not there, if you have basic CSS knowledge, you should be able to do it with no problem. If you don't, then I suggest you ask someone who does to help you out!