I am trying to edit the look of the xsell products heading.

All other headings on my site have a dark background image with white text. Without the background image showing, you cant see the text and the xsell products just look like they have been plonked on the page.

example:
Table.infoBoxHeading {
background: url(../../images/catheader.jpg)no-repeat;
height: 40px;
background-color:#AA1331;
}

I have tried for the life of me to find where/how I an add a css style to the xsell header but have had no luck.

Where can I edit this and how?

I have tried:

<!-- xsell_products //-->
<?php
$info_box_contents = array();
$info_box_contents[] = array('align' => 'left', 'params' => 'class="xsell"', 'text' => TEXT_XSELL_PRODUCTS);
new contentBoxHeading($info_box_contents);


CSS:

.xsell {
background: url(../../images/crumbheader.jpg)no-repeat;
height: 40px;
background-color:#AA1331;
padding: 5px;
}

Help please