i don't know if this can be fixed with a MOD or where to put it, but it you notice, if you make the browser bigger, the forum stretches. If you go on a site like www.cloverfieldmovieforum.com (just using as example) it just shows the background color if you stretch it. Is there a way I can make it so that my forum shows the background color if stretched.
How do I set the forum width? (http://docs.simplemachines.org/index.php?topic=1021.0)
Or you could try using a fixed width theme from the themes site.
What theme are you using? Can you attach the index.template.php file for that theme here? It probably has a fixed width set.