3.8 branch: Undo the colors.css changes in [26938] and move the rules to wp-admin.css.

This will prevent us from shipping a full re-build of the color schemes in 3.8.1.

see #26699.

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git-svn-id: http://core.svn.wordpress.org/branches/3.8@26899 1a063a9b-81f0-0310-95a4-ce76da25c4cd