It doesn't use css.
It appears to use some txt file with a multi-dimensional array in what appears to be php.
If I append to the existing txt file in the style_vars array like so below. The image_header class doesn't show up anywhere when I look inside the style section (click on edit and open all sub menus) after uploading it.
I don't believe I would be the first person to ask such a question.
'image_header' => 'background-image:url("http://www.foo.com/threads/images/forumimages/foo.gif");
background-repeat:no-repeat;
height: 360px;
width: 130px;',
