I suggest creating or modifying your existing classes to utilize the classes auto generated by the CMSListMenu.  Nice thing is you can specify a comma separated list of node prefixes.  For instnace: first, second, third.  Then you have a structure like so:
<ul class="firstCMSListMenuUL">
  <li class="firstCMSListMenuLI">
    menu option 1
  </li>
  <li class="firstCMSListMenuLI">
    menu option 2 w/nested items
    <ul class="secondCMSListMentUL">
      <li class="secondCMSListMenuLI">
        Nested item under menu option 2
      </li>
    </ul>
  </li>
</ul>
 You can easily create styles for it in your css.  
Another option is to uncheck the box so it will not render the CSS classes dyamically for you.