Editing Styles.css

Please have a look at the site I am working on at:

http://www.stlouisrealestatelaw.com/

I want to make a 100px border on the left and right. I added the code to do this to the body element in the stylesheet, but as you can see the main area still spreads across the entire page. Can anyone see where in the stylesheet I would need to add margin values to fix this?