How do I change the background beyond a max-width without having to change all of my pages?

Allow me to explain further... I currently have a max-width set for the body on my .css file. That works fine, so I do not want to change it. The problem is that the whole page looks so stark now, and for higher resolutions it's a big white space to the right of the max-width. I realize I can change the body background color, add a div cl and put divs around the content of all the pages. The problem is that this would be time consuming, and I have things installed such as gallery2 and wordpress that seem a bit tricky to override. I should also mention that all my pages have a php extension, and I'm using php includes to add my header/menus to all the pages. That causes the main .css file to affect everything, even if it is not linked to that particular page. Before I start changing every page, I would like to know if there's an easier way to change the background of the area to the right of the max-width, where the content ends. Any ideas would be appreciated. Thanks so much!