[ticket/12562] Add max-width to proSilver

Let's try this again.

http://area51.phpbb.com/phpBB/viewtopic.php?f=108&t=45739&start=70#p266871
https://tracker.phpbb.com/browse/PHPBB3-12562

![lasttry2](https://cloud.githubusercontent.com/assets/3015760/3449856/f169300c-016d-11e4-97e0-ce962f21bd65.png)

The response has been very positive so far.
PHPBB3-12562
I think we should only hide the border when we switch to the mobile header, so we have only "2 designs" not 3