It's going to take some work. Right now, the side borders come from a single background image that covers the full width of the page:
http://www.tarantuladatabase.com/assets/fo.../content_bg.jpg(This also forces a fixed-width design, which has its own problems.)
You'll need to create a new background image with just the border pattern you want, and use it as the background image for the part of the page you want. (I think you can use the new background image for div.forabg and change the background back for ul.topiclist.forums
FWIW, that markup would be a lot cleaner as a table, rather than the mess of CSS-dependent div soup that it is now.