[ticket/12598] Improve search box styling

https://tracker.phpbb.com/browse/PHPBB3-12598

18-9-2014 tests:
- [x] FF 32, normal, responsive, RTL
- [x] Chrome 37, normal, responsive, RTL
- [x] Opera 23, normal, responsive, RTL
- [ ] ~~Safari 5.1.7, normal, responsive, RTL (doesn't support font-size: 0)~~
- [x] IE11, normal, responsive, RTL
- [x] IE10, normal, responsive, RTL
- [x] IE9, normal, responsive, RTL
- [x] IE8, normal, "responsive", RTL
- [x] Android Chrome, responsive, RTL

![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