[ticket/12608] Improve notifications drop-down menu styling in header

https://tracker.phpbb.com/browse/PHPBB3-12608
http://area51.phpbb.com/phpBB/viewtopic.php?f=81&t=45657

PHPBB3-12608
Can we remove the over-specificity here and simply go with `.notification-reference, .notification-location` and so on? :)