Fixed #32018 -- Extracted admin colors into CSS variables.

Defined all colors used in the admin CSS as variables. Implemented the
following standardizations and accessibility improvements while at it:

- Improved the contrast of text to not use ratios of less than 3:1 anymore.
- Most hover states already used desaturated and darkened colors.
  Changed object tools to follow the same rule instead of showing the
  primary color on hover.

Various places used similar colors; those have been merged with the goal
of reducing the count of CSS variables. Contrasts have been improved in
a few places.

- Many borders used slightly different colors (e.g. #eaeaea vs. #eee)
- Help texts used #999, this has been changed to --body-quiet-color
  (#666) which has a better contrast.

Introduced fast color transitions on links and buttons.

docs/releases/3.2.txt

@@ -117,6 +117,8 @@ Minor features
 * Read-only related fields are now rendered as navigable links if target models
   are registered in the admin.
 
+* The admin now supports theming. See :ref:`admin-theming` for more details.
+
 :mod:`django.contrib.admindocs`
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