Begin simplifying some of the visual elements in the admin, starting with gradients. White space purposefully not currently altered. props lessbloat, melchoyce, aaroncampbell, sabreuse. see #23415.


git-svn-id: https://core.svn.wordpress.org/trunk@23407 1a063a9b-81f0-0310-95a4-ce76da25c4cd