fix style of public share page

Old:
![public-share-old](https://f.cloud.github.com/assets/245432/1429029/399ab07c-40ac-11e3-9b15-548b54cdb3b1.png)
New:
![public-share-new](https://f.cloud.github.com/assets/245432/1429033/3da880cc-40ac-11e3-974b-37a443501a42.png)

Was part of #5418 which isn't merged.

Tested in IE8, Firefox and Chrome.

cc @owncloud/designers