[ticket/12598] Improve search box styling

https://tracker.phpbb.com/browse/PHPBB3-12598

![newsearch](https://cloud.githubusercontent.com/assets/3015760/3170466/20e332d6-ebb8-11e3-9292-99e7dc54a809.png)
![newsearch](https://cloud.githubusercontent.com/assets/3015760/3375561/566e93fa-fbcb-11e3-8569-8a1ad125ef0e.png)
PHPBB3-12598
I'll add some documentation for the `*` and `\9` hacks we currently use.

That way this PR can be merged without yet more testing delays.

We'll just make a ticket specifically for removing legacy hacks and tweaks.