I hope someone can figure this out - it seems to me there ought to be a simple solution, but for the life of me I can't figure it out.

I recently upgraded to 2.0.18 and re-did the design of my site. I am using Newbbex 1.4 for my message boards and a fixed-width theme. The forums look fine when reading messages, but when you post a new topic, the editor is too wide for the frames and overlaps the right side (see image link). Is there a way to modify the editor so that it doesn't do this, or to prevent it? The image is a bit small, but the blue area is the selected text so you can see how far it overlaps. It's more of a nuisance than anything, but I'd love to fix it.

Thanks in advance.