I'm guessing there would be something in the theme folder to change... admin/styles... probably a css in that directory
I would be looking for a theme folder somewhere in that area.
I'm more familliar with other CMS/BB's out there, but the concept is basically the same with all of them.

However in your control panel (CP) for the BB, there might be an editor for the CSS already there, this might be linked directly to the theme itself in your CP... might be easier to use than your notepad. But likely this correction you want is a PHP edit. So I'd look for a Item in your CP to do this first before messing around directly with the php pages.