[3.2][WIP][ticket/12654] Improve header search-box styling

http://area51.phpbb.com/phpBB/viewtopic.php?f=108&t=45679
https://tracker.phpbb.com/browse/PHPBB3-12654

![lalala](https://cloud.githubusercontent.com/assets/3015760/3181212/070ab062-ec3c-11e3-9609-3e2d5714ca9e.png)

Depends on: https://github.com/phpbb/phpbb/pull/2504

This is a fork of the #2504 branch. I'll rebase this on develop-ascraeus after that one is merged.