I just included a link to the main stylesheet I use for this site, and then wrapped the entire thing in my page-wrap I use. I made almost zero modifications to the phpBB styling itself.
One little hint on designing around phpBB though, is that is caches the design files. Like "overall-header" and "overall-footer". So everytime you change one of these files, you gotta go in and deleted the cache file before you refresh your browser or you won’t see the changes.
Would it be possible to make the forum-header (//css-tricks.com/images/forums-header.jpg) smaller, or just remove it? Then the forum would move a bit further up on the page. Gets somewhat tiresome to do alot of scrolling to get to the content ;)