Dark mode support

[stenno]

As it was a request, we should try to implement OS-defined dark mode/night mode support.
This can be done by css:
@media (prefers-color-scheme: dark)
See: https://developer.mozilla.org/en-US/docs/Web/CSS/@media/prefers-color-scheme

[stenno]

I can implement that, and play around with a few color schemes.