Add Dark Mode

[hoechenberger]

Would be cool to support a "dark mode"; Safari, the default browser on macOS and iOS, will then automatically request the "dark" website when Dark Mode is enabled on OS-level (DM is expected to ship with the next major iOS release). It's something to play with and without any real benefit in our case, but hey, would be sooo cool, wouldn't it ;)

https://webkit.org/blog/8840/dark-mode-support-in-webkit/

There's an implementation for React using "hooks", a relatively new feature (don't know if we'd have to update some of our packages to get this to work):
https://github.com/donavon/use-dark-mode

But there's probably also many more solutions. This is just one of them.