According to this from FM layout, the background takes its color, and properties from color_module_border, and color_page_title

#top-color { position: fixed;
top: 0;
left: 0;
z-index: -1;
width: 100%;
height: 25em;
background-color: $*color_module_border;
border-bottom: .5em solid $*color_page_title; }


You can change the properties within the layer by removing that part, but you'll also lose the background color to the #header. For that, you can add in teh css box:
#header {
background:url("http://static2.grsites.com/archive/textures/dgren/dgren019.jpg") repeat scroll 0 0 transparent;
}

Does anybody else have any ideas?