That piece of code isn't in the external css on your site.
If you look at the source code on any page of your site close to the top there will be a section that reads
Code:
/* Header cells - the blue and silver gradient backgrounds */
th {
color: #797979; font-size: 11px; font-weight : bold;
background-color: #B41B47; height: 25px;
background-image: url(templates/subSilver/images/#000000);
}
#B41B47 is what you need to change but seeing as I don't have any experience with forum modding I couldn't tell you where to find it in the admin panel.
