First off, to get it to the top left corner of the screen you would need to remove "text-align: center;" and "margin-top: 50px;" from the #body.
As far as the rest, you would pretty much have to change the widths from a certain px to 100%. That is a very nice and clean layout. The index page seems a little small for my taste. I assume that is why are asking to how to change it to be full screen.