Dark mode: fix contrast of footer highlight

[hugovk]

Fixes#161.

Before

After

No change to light mode

Preview

https://python-docs-theme-previews--162.org.readthedocs.build/en/162/library/gettext.html

[ferdnyc]

Hah! I was just coming here to get started on a similar pull request.

Unfortunately, while this is merged, it still isn't deployed on docs.python.org. Those docs still show the unreadable yellow background, in the dark theme.

Is there a procedure for getting the live docs to pull in theme updates?

[hugovk]

Hah! I was just coming here to get started on a similar pull request.

Thank you!

Is there a procedure for getting the live docs to pull in theme updates?

Yes, the procedure is:

• Release the theme (the last was in October: https://github.com/python/python-docs-theme/releases)
• Wait 24-48 hours for the docs to rebuild (they can use the latest, but I can't remember if it caches dependencies: https://github.com/python/cpython/blob/b69548a0f52418b8a2cf7c7a885fdd7d3bfb1b0b/Doc/requirements.txt#L19)

We have a few thing pending release, I'll make one later today or this weekend.