Presumably this is on the What's New page?
Look in the find_new_threads template and the associated thread_list_item template, the faint CSS class on this line:
<a{xen:if {$visitor.user_id}, ' href="{xen:link threads, $thread}"'} class="faint"><xen:datetime time="$thread.post_date" title="{xen:if {$visitor.user_id}, '{xen:phrase go_to_first_message_in_thread}'}" /></a><xen:if is="{$showForumLink}">,