the wrapper for your main content in wordpress, has the CSS, for that part, got any overflow? Adding overflow: visible or auto makes your grey background appear, but also shows a scroll bar at the edge of that wrapper.
the best thing to do is to put in a comment, and see what html code is generated (view > source), then see what id’s/classes are being used, and then style all of them in your CSS file (this will help in the long run, cause then when wordpress updates, nothing breaks, adding in your own id’s &/or classes will be lost on an upgrade).
any & all styling should be done on your CSS file
also I see you have not cleared your floats, this will be your problem to your background.