Creo que cambiando tu forumbit_depth1_cat por esto ya debería de ir:
Código:
<div class="thead_main"><div class="thead_left"><div class="thead_right"><div class="thead_text">
<strong>{$lang->sub_forums_in}</strong>
</div></div></div></div>
<table border="0" cellspacing="{$theme['borderwidth']}" cellpadding="{$theme['tablespace']}" class="tborder">
<tr>
<td class="tcat" width="2%"> </td>
<td class="tcat" width="59%" colspan="2"><span class="smalltext"><strong>{$lang->forumbit_forum}</strong></span></td>
<td class="tcat" width="7%" align="center" style="white-space: nowrap"><span class="smalltext"><strong>{$lang->forumbit_threads}</strong></span></td>
<td class="tcat" width="7%" align="center" style="white-space: nowrap"><span class="smalltext"><strong>{$lang->forumbit_posts}</strong></span></td>
<td class="tcat" width="15%" align="center"><span class="smalltext"><strong>{$lang->forumbit_lastpost}</strong></span></td>
</tr>
{$forums}
</table>
<div class="tfoot_main"><div class="tfoot_left"><div class="tfoot_right"></div></div></div>
<br />
Puedes ver que lo único que añadí fue: colspan="2"