[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
They should use the same class name. This only causes confusion and is rather ugly. Prefix the selector with the appropriate parent if there's a need to have different styling.