If using the mega menu addon, try this...
In skin/reboot/custom/header/header_mega_menu.tpl
Replace...
Code:
<li><a href="{if $c.childs}#sub{else}home.php?cat={$c.categoryid}{/if}"{if $c.childs} class="arrow"{/if}>{$c.category}</a>
With...
Code:
<li class="cat_{$c.categoryid}"><a href="{if $c.childs}#sub{else}home.php?cat={$c.categoryid}{/if}"{if $c.childs} class="arrow"{/if}>{$c.category}</a>