As described in the readme, find this section in tpl_categories.php
PHP Code:
      // categories dressing - add divider above specified cats
      
if(in_array($current_pathexplode(",",'3,22,21_56'))) { //replace numbers with your cat ids separated by commas
        
$content .= '<hr class="catBoxDivider" />' "\n";
      } 
and put the ids of all of your top categories in the list.