Before giving you a CSS answer, I would ask you tp take a good look at the stylevar system for vBulletin. Some good playing around time with the stylevar system should yield you some good results. I will post some references that will give you a hand.
If you decide you need CSS help then we need a link to your board, unless you are using the default vBulletin style.
Basic:
http://www.vbulletin.com/docs/html?m...rsion=40201500
https://vborg.vbsupport.ru/showthread.php?t=256370
http://www.vbulletin.com/vbcms/conte...-10-Easy-Steps
More Advanced:
https://vborg.vbsupport.ru/showthread.php?t=230309
More Advanced - Videos:
http://www.youtube.com/watch?v=sfwle1enLms