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.
Built from https://develop.svn.wordpress.org/trunk@26482


git-svn-id: http://core.svn.wordpress.org/trunk@26379 1a063a9b-81f0-0310-95a4-ce76da25c4cd