There are quite a few CSS files you'd need to edit and at the moment I don't remember them as it was a while ago I did it. Google is your friend when it comes to that though. In fact I think I found various tutorials on how to do it.
You won't need to reinstall the whole of PHPBB but you will need to update the theme you are using for it.
It might be best for you to pay someone else to do it for you if you are in a hurry.