On styles demo you have a fake unread forum, which is a forum with id 12. It shows icon of unread forum, but forumrow.S_UNREAD_FORUM is missing.

Example: http://www.phpbb.com/styles/demo/3.0/index.php?style_id=24

Source code for unread forum link shows this

<a class="forumlink" href="./viewforum.php?style=24&amp;f=12">Unread Forum</a>

but code in template is this

<a class="forumlink<!-- IF forumrow.S_UNREAD_FORUM --> link-new<!-- ENDIF -->" href="{forumrow.U_VIEWFORUM}">{forumrow.FORUM_NAME}</a>