[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
This should point out that it's a CSS class that we're expecting. May even want to rename to method to `get_style_class`.