All Shoved Left

I'm sorry to be a pain, but I'm getting no help with this on Forumotion, where my PunBB forum is. They haven't specified what version of PunBB I've got, so forgive me if I'm in the wrong spot.

This is the problem: http://writerswell.forumotion.com/

All the tables and everything have been shoved left, and I can't figure out how to centre them again. Can any of you help me? I've tried changing the margins. That just makes it wobble when I refresh the page, then the tables jolt back to the left as if they're nailed there.

I've tried rewriting the CSS elsewhere, then copying and pasting it into the CSS box. That does nothing. I'm not sure how I did it, either. I know it's my own fault, but I think it looks silly and would like to get it all back in the centre.