[Admin]

Why can't you create a new template set, assign it to a style and use that style in only that forum? Easier than hacking.

Or hack forumdisplay:

    // there are threads
    eval("\$forumdisplay[threadslist] = \"".gettemplate('forumdisplay_threadslist')."\";");

Change to something like:

    // there are threads
    eval('$forumdisplay[threadslist] = "'.gettemplate(iif($foruminfo['forumid'] == 3, 'forumdisplay_threadslist_specialversion', 'forumdisplay_threadslist')).'";');