It's driving me nuts too, you have 3 stylesheets.

Find in your stylesheet_new.css;

Code:
/*wrappers - page or section containers*/
#mainWrapper {
	background-color: #ffffff;
	text-align: left;
	width: 750px;
	vertical-align: top;
	}
Change the value in red to: 100%;

I'd recommend an upgrade to the newest version. Perhaps a bit of work to do, but far less work than trying to battle three stylesheets and having needed patches wiping out that obsolete template.