Adding a class to tag list in a function

If you are passing the before parameter that you can simply add any class that you want. You then style that class in style.css.

<?php
    if ( get_the_tag_list() ) {
        echo get_the_tag_list('<ul class="tag-list"><li class="tag-item">','</li><li class="tag-item">','</li></ul>'); 
    }
?>