The header and the footer are both inside the Body.
Look at the CSS applied to the header - perhaps you want something similar for the footer?
Also, you could set the body to be 100% wide - I suspect it currently has margins set or something.
So, the goal is not to move the footer outside the body div - the right goal is to correct the css for the body div, or the footer, to get the effect you want.
cheers
Bruce