How exactly does a wrapper work?

I wan’t to edit Gluttony theme. At the end of header.php are two unclosed div elements. One is a wrapper and the other is the container(in which the main-column and content reside. See: http://i262.photobucket.com/albums/ii114/adredz/wrapper.jpg, the outmost red border is the container, yellow is for the maincolumn, and green is the content). If I close then wrapper, the container slides to the left and can’t be centered through the use of margin. If I close the container tag, it disappears. I’m very confused. I just wan’t to make the content extend to both sides so it would overlap the container. How would I do it? I tried editing style.css with no luck…