History: Full-width backgrounds and fixed-width content

This is extracted out of the FiveAlive theme stylesheet (fivealive.css), for appearance and behavior as in that theme: header and footer backgrounds go full-width, but header content, page middle content, and footer content are fixed-width. I did a quick check but there could be issues but anyway it's a starting point. Yellow is used as a temporary background color to check the performance and to be replaced by the actual background color/image. (Please make additions or changes as you like.)

If the full-width treatment is needed for just one of the header or footer, then don't use the rules for the area that should be full-width.

(If the theme is for a particular website, then there's not so much complexity regarding the fixed-width size, as the width can set it in the stylesheet and/or in Look and Feel admin. When making a theme for distribution, it can be a little tricky to set a default theme fixed-width size rule that works but isn't so strong that it prevents the Look and Feel setting from being respected.)