Themes: Prepare theme.css for a merge with wp-admin.css.

* Let the code breathe.
* Add necessary vendor prefixes.
* Remove .current-label:before since it was hidden.

see #25966.

git-svn-id: https://develop.svn.wordpress.org/trunk@26482 602fd350-edb4-49c9-b593-d223f7449a82