told you already.
yoursite.com/forum/
clientscript/vbulletin_css (if you store them in files ofcourse)
You may have multiple .CSS files there so look in source of your site and notice the exact css file that is imported in the code like this
HTML Code:
@import url("clientscript/vbulletin_css/style-e8casdas63-0007.css");
Then simply edit what you want and upload/overwrite.