To make your site take up the whole browser you need to use the /public_html/templates/tmpl_uni/css/general.css file.
Look for .main and change the width function to width:100%;
This will make your site take the whole page up, you will however need to change your whole layout to accomodate this.
Don't forget to copy the general.css file before you modify it.

Hope this is what you wanted to know