how to add background behind - not in -center area
 

I don't know where to post this and I can't find a setting for it. I want to insert a background image that would affect everything but the center post/page area. I want my blog to match my HTML pages. So, I want http://www.ellenfinkelstein.com/pptblog/ to match http://www.ellenfinkelstein.com. In the latter, you'll see a blue gradient background image and that's what I want for my blog.