You have done some strange editing that makes it impossible to control the page in the normal way.
You need to delete the
width: 1000px;
from the body {} rule so that the body can expand to fill larger monitors. Until you do that, no background-color will show.
You have somehow terminated the #mainWrapper div that is supposed to enclose your entire page after the header, and then enclosed the rest of the page in a series of five nested <font> tags. Delete those font tags and the </div> that prematurely closes your mainWrapper; then your #mainWrapper width setting of 960px will apply to the whole page.
How did you get those font tags there, anyway? And why?
HTML Code:
<li><class="float-right"><font size="2" /><form name="quick_find"
Also, <li><class="float-right"> is invalid HTML. If you want to give a class to the <li>, do it like this: <li class="float-right">
Or if you are trying to float the form, apply CSS styling to that, not as a separate <class> tag floating in limbo...