Merge colors.css into wp-admin.css. Non-default color schemes became overloads rather than swaps in the 3.8 redesign. Benefits include:

* No more border-width, border-style, border-color insanity.
* Point releases are much less likely to require extra finagling to avoid rebuilding the color schemes.
* Yours truly has a better overall vision of ~14,000 lines of admin CSS and where we go from here.

1,065 net lines of red, y'all.
props helen, jorbin. fixes #18380.

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