CSS tip for footer

Hi

I am trying to understand CSS. Actually I am looking into a file I bought but I can’t fix the footer the way I want it to show.

The problem is that in big screens like mine (24") after the footer that is grey I see white, and I want the grey to stay until the end of the browser window.

You can see it here:

http://www.profebox.com/admin/index.html

my #footer css is:

#footer {
clear:both;
float:left;
width:100%;
display:block;
height:200px;
position:relative;
}

I’ve tryed to set the height 100% but it doesn’t work. Any tip on how to solve this?

Thanks.