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.

Built from https://develop.svn.wordpress.org/trunk@26072


git-svn-id: http://core.svn.wordpress.org/trunk@25992 1a063a9b-81f0-0310-95a4-ce76da25c4cd