We could figure it out pretty quickly with a link, but if your site is on a test server we can't see, we just have to guess. And my guess is that you've got the background color inside something which also needs the width set to 200px. Search the stylesheet for "width:" and make sure you know what each of them is doing.

Do you use Firefox as your development browser? With some of those great developer's tools added in to it? If so, Aardvark can tell you which element the background color is really in.

HTH

Rob