Raimond -
The code you show is from the older version (v2.2) of Categories Dressing, and will most likely not work in the latest version (v2.6). What version do you have? The comments at the top of tpl_categories.php will say this. If you have the older version, you need to replace it with the new version for this job.

If you are not understanding the full explanation, it's ok, you don't need to. Just add this to the bottom of your stylesheet_categories_dressing.css:
Code:
#categories ul+li.cat-top {
    margin-top: 1.0em;
    border-top: 1px solid #aabbcc;
    }