Forgot to note that the above code is in /includes/modules/your_template/categories_tabs.php.
If you want to put the class tag in the <li>, you can leave that file as is and edit /includes/templates/your_template/templates/tpl_modules_categories_tabs.php instead.
PHP Code:
<div id="navCatTabs">
<ul>
<?php for ($i=0, $n=sizeof($links_list); $i<$n; $i++) { ?>
<li><?php echo $links_list[$i];?></li>
<?php } ?>
</ul>
</div>
Add
<?php echo ereg('(category_subs_selected)',$links_list[$i])? ' class="category_top_selected"':''; ?>
PHP Code:
<div id="navCatTabs">
<ul>
<?php for ($i=0, $n=sizeof($links_list); $i<$n; $i++) { ?>
<li<?php echo ereg('(category_subs_selected)',$links_list[$i])? ' class="category_top_selected"':''; ?>><?php echo $links_list[$i];?></li>
<?php } ?>
</ul>
</div>
If this code finds the string "category_subs_selected" inside the link being processed, it will add the class tag to the <li>.