Add support for dark theme/mode

It would be good to add support for a dark theme. However, the current theme is good enough on the eyes.

I think it is not a super important. One can simply use a dark mode extension. 😅

No files, tests, or entry points are named. First inspect the site's CSS and theme structure, then determine the scope and visual requirements for a dark mode. Done means the project has a consistent dark theme that can be checked across the site's pages.

Written by the indexing model from the issue text.