[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

__edit:__ design changed, as requested.

__edit 2:__ better screenshot
![again](https://cloud.githubusercontent.com/assets/3015760/3470314/ff0134fc-02b8-11e4-8ed3-cf746ceddb95.png)

The response has been very positive so far.
PHPBB3-12562