I'm using the twenty twelve theme and in the admin panel under appearance, there is an option to change the background color through a color picker or by uploading an image (http://en.support.wordpress.com/themes/custom-backgrounds/).
My question is, how does one go about making the background two colors? I'd like the section at the top of the browser window down to the bottom of the header to be one color and then from the bottom of the header to the very bottom of the browser window to be another color. The effect I'm trying to achieve is as if the header is across the entire browser window, similar to this site: http://uncrate.com/
For clarification, I'm referring to the background that is outside of the actual blog area. In the twenty twelve theme, the default is grey (#e6e6e6).