Different backgrounds behind header area and body area?

This is a little hard to describe, which may be why I haven't found anything about it in the support forums after half an hour of searching. I am working on a website for someone who wants the parchment background image that you can see in the site-content section to run all the way across the page -- so if you're looking at the site on a monitor with a wide screen, you won't see the dark grey at all at either side of the content, just the parchment. However, the header would just have the dark grey on either side of the header image, and NOT the parchment.

Is there a way to set a background image so it covers the whole page starting underneath the nav bar, leaving the areas to the right and left of the header image without that image in the background, so you only see the background color?