Say hello to a fresh new look for the WordPress admin.

Still to come: more color schemes, a responsive component, and more.

see #25858.
props iammattthomas, tillkruess, EmpireOfLight, melchoyce, ryelle, joen, mitchoyoshitaka, sirbrillig, andypeatling, isaackeyet, Otto42, dd32, matt, helen.


git-svn-id: https://develop.svn.wordpress.org/trunk@26072 602fd350-edb4-49c9-b593-d223f7449a82