Dark mode option (off by default) for documentation website

[AnnaKornfeldSimpson]

The theme Tasks uses for the documentation website supports dark mode. Here is a page discussing it, which also has an example button for swapping light/dark and Javascript to handle the swap. It would be great if such a button could be added to to the Tasks documentation site also to give users the option (off by default) of reading the docs in dark mode.

[claremacrae]

A pull request would be welcome, if anyone was interested in doing this.