[ticket/12598] Improve action-bar search box styling

https://tracker.phpbb.com/browse/PHPBB3-12598

![newsearch](https://cloud.githubusercontent.com/assets/3015760/3170466/20e332d6-ebb8-11e3-9292-99e7dc54a809.png)

PHPBB3-12598
`.inputbox` already defines this. Why is it necessary to set it again here?