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.