Add Background Image to Scholar Theme?

I would like to add a background image to this theme. I understand the basics of the css background property, but am not sure where I should set this image given the various sections of the theme (header, navigation, content area etc). Is it possible to have the background image start at the very top of the page and span down through the menu area, site description and the main content area? Or can I only set a background image for each section?

Thanks for any guidance!