You will need to follow the directions in the comments near the top of tpl_categories.php (where it says "A.category-holiday") to make a new tag for each subcat you want to be different from standard, using the full cPath in the case.
PHP Code:
// to make a specific category stand out define a new class in the stylesheet example: A.category-holiday
// uncomment the select below and set the cPath=3 to the cPath= your_categories_id
// many variations of this can be done
//      case ($box_categories_array[$i]['path'] == 'cPath=3'):
//        $new_style = 'category-holiday';
//        break; 
For a subcat whose cPath is 3_42, make it
PHP Code:
      case ($box_categories_array[$i]['path'] == 'cPath=3_42'):
        
$new_style 'category-sub-3_42';
        break; 
and then style

.category-sub-3_42-text { background-color: #aabbcc;}

or whatever you want.