Fix diff colours in docs (#612)

#582 upgraded Furo,
along with it's new accessible-pygments dependency
(pradyunsg/furo@be0072d).

Unfortunately, accessible-pygments doesn't support the `diff` code block
and is missing some CSS
(Quansight-Labs/accessible-pygments#31), so
diffs look like:

<img width="756" height="230" alt="image"
src="https://github.com/user-attachments/assets/96eb6002-f162-4128-8920-fdc8be8c96d5"
/>

<img width="746" height="224" alt="image"
src="https://github.com/user-attachments/assets/ef9bc771-ba58-49f9-899a-12564959945c"
/>

https://django-upgrade.readthedocs.io/en/latest/fixers.html#django-4-2

Instead, set our own colours:

<img width="748" height="227" alt="image"
src="https://github.com/user-attachments/assets/fa5e1855-dcdf-44c0-9083-d048eb866896"
/>

<img width="752" height="232" alt="image"
src="https://github.com/user-attachments/assets/5c57febb-4b3a-45fe-8ce8-93e45b96ea24"
/>

I've confirmed these all exceed the WCAG AA min ratio.

Author: hugovk

docs/_static/custom.css

@@ -0,0 +1,9 @@
+.highlight .gd { color: #A40000 !important; }
+.highlight .gi { color: #116600 !important; }
+
+@media (prefers-color-scheme: dark) {
+    body:not([data-theme="light"]) .highlight .gd { color: #FF6B6B !important; }
+    body:not([data-theme="light"]) .highlight .gi { color: #69DB7C !important; }
+}
+body[data-theme="dark"] .highlight .gd { color: #FF6B6B !important; }
+body[data-theme="dark"] .highlight .gi { color: #69DB7C !important; }

docs/conf.py

@@ -60,6 +60,8 @@
 # a list of builtin themes.
 #
 html_logo = "_static/logo.svg"
+html_static_path = ["_static"]
+html_css_files = ["custom.css"]
 html_theme = "furo"
 html_theme_options = {
     "dark_css_variables": {