[christindall]

Ok - so, figured it out. If anybody else cares, here's what I did:

First, I created a new file in light_responsive -> customer and named it mobile_categories.tpl

The contents of the file look like this:

{if $categories_menu_list ne '' or $fancy_use_cache}
{capture name=menu}

    {foreach from=$categories_menu_list item=c name=categories}
      <li{interline name=categories foreach_iteration="`$smarty.foreach.categories.iteration`" foreach_total="`$smarty.foreach.categories.total`"}><a href="home.php?cat={$c.categoryid}" title="{$c.category|escape}">{$c.category|amp}</a></li>
    {/foreach}

  {assign var="additional_class" value="menu-categories-list-simple"}

{/capture}

    <ul>
      {$smarty.capture.menu}
    </ul>

{/if}

Then, in mobile_header.tpl, I added the following code immediately after the first ul:

    <li class="dropdown">
      <a id="main-menu-toggle" class="dropdown-toggle" href="#">
        <span class="fa fa-th-list"></span>
      </a>
      <div id="categories-menu-box" class="dropdown-menu">

        {include file="customer/mobile_categories.tpl"}
        

      </div>
    </li>

the span class fa-th-list - I chose that because I liked that it was similar but different to the "list" icon - you can see an entire list of options at http://astronautweb.co/snippet/font-awesome/ - the file that references that class is in skin -> common_files -> css -> font-awesome.min.css


I think that's it. when you "shrink" your desktop screen, or look at it on a phone screen, you'll see the extra dropdown nav full of categories. Now, my site doesn't have that many root categories - I have NO clue how this will work if you have tons of root categories (although I'll know in a few weeks when I upgrade my main site to 4.7.2, as I have like 100 root categories on that site...)