I think the reason you can't find what your looking for is because thats a forums theme template. If you want to alter the header, you need to be looking at the files in the sites root such as header.html, index.php, etc.

BTW, I was just looking at your site and it loox like your well on your way to fixing the problem. When in doubt, just create an imagefile that will take up your entire header size. I would caution you though that if you have your site set to autostretch rather than locked pixels, your header will stretch your page at lower resolutions and justify left at higher resolutions.