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/ref/contrib/admin/index.txt

@@ -2802,6 +2802,34 @@ creating your own ``AdminSite`` instance (see below), and changing the
 :attr:`AdminSite.index_template` , :attr:`AdminSite.login_template` or
 :attr:`AdminSite.logout_template` properties.
 
+.. _admin-theming:
+
+Theming support
+===============
+
+The admin uses CSS variables to define colors. This allows changing colors
+without having to override many individual CSS rules. For example, if you
+preferred purple instead of blue you could add a ``admin/base.html`` template
+override to your project:
+
+.. code-block:: html+django
+
+    {% extends 'admin/base.html' %}
+
+    {% block extrahead %}{{ block.super }}
+    <style>
+    :root {
+      --primary: #9774d5;
+      --secondary: #785cab;
+      --link-fg: #7c449b;
+      --link-selected-fg: #8f5bb2;
+    }
+    </style>
+    {% endblock %}
+
+An up-to-date list of CSS variables is at
+:file:`django/contrib/admin/static/admin/css/base.css`.
+
 ``AdminSite`` objects
 =====================