Sorry but what you explain in the first part I don't understand..
If I open portal_announcement template I only can edit him so what do you mean???
Edit the content inside that template and add new class to table tag or just copy following content into that template
<table cellspacing="{$theme['borderwidth']}" cellpadding="{$theme['tablespace']}" class="portal_announcement tborder">
<tr>
<td class="thead"><strong>{$icon} <a href="{$mybb->settings['bburl']}/{$announcement['threadlink']}">{$announcement['subject']}</a></strong></td>
</tr>
<tr>
<td class="trow2" align="right">
<span class="smalltext">{$lang->posted_by} {$profilelink} - {$anndate} {$anntime} {$numcomments}</span>
</td>
</tr>
<tr>
<td class="trow1">
<table border="0" cellpadding="{$theme['tablespace']}" width="100%">
<tr>
{$avatar}
<td class="trow1">
<p>
{$message}
</p>
{$post['attachments']}
</td>
</tr>
<tr>
<td align="right" colspan="2" valign="bottom">
<span class="smalltext">
<a href="{$mybb->settings['bburl']}/printthread.php?tid={$announcement['tid']}"><img src="{$theme['imgdir']}/printable.gif" alt="{$lang->print_this_item}" title="{$lang->print_this_item}" /></a> <a href="{$mybb->settings['bburl']}/sendthread.php?tid={$announcement['tid']}"><img src="{$theme['imgdir']}/send.gif" alt="{$lang->send_to_friend}" title="{$lang->send_to_friend}" /></a>
</span>
</td>
</tr>
</table>
</td>
</tr>
</table>
<br />
Great, Now I know what you mean sorry....
But works great!!! Thank!!