It all depends on what you want to modfiy. If it's simple colour changes then you can tinker around in the
admincp >>
styles & Templates and play with the css a little untill you get a more desireable colour scheme. On the otherhand, if you are wanting to make more extensive changes like change the graphics you'll first need to have access to the PSDs and make the changes via a graphics editor and/or replace images and overwrite them if you have aquired another image that you want to replace the original with.
You may want to pay vbulletin.org a visit and browse through their styles I'm sure there are a few valentine styles over there and I'm sure I have some valentine emoticons (smilies) at my site.
