Enable automatic light/dark theme switching based on system setting

New MacOS and Safari support a system-wide "Dark Mode", and make this available through CSS media queries with the prefers-color-scheme media feature. This is likely to start being supported in other browsers before long, so it would be good to be able to support automatic switching between light/dark themes for people that like that.

Some more info in this post: https://paulmillr.com/posts/using-dark-mode-in-css/