question: with what do you want to fill the background?
(technically speaking, it is already filling the whole background as you have horizontal and vertical repeat)
your background.gif 5 * 250px) has this gradient, which ends in the serrated edge and a transparent lower quarter end.
do you want the lower page to have the same color as the edge of your gradient?
if so, try and edit the body background in your style.css:
margin:0 120px 0 120px;
background: #a7a1c7 url('images/background.gif') repeat-x;
font-size: 62.5%; /* Resets 1em to 10px */
font-family: 'Lucida Grande', Sans-Serif;
refresh your browser afterwards, or reload the page, to make the change visible.