Respect users' preference for light/dark mode

[cormullion]

Coming soon (or even already here) to a browser near you, a way of choosing to display light or dark mode:

https://drafts.csswg.org/mediaqueries-5/#prefers-color-scheme

Is this something that can be added to Documenter?

[mortenpi]

Documenter's theme switching is JS-based, but that shouldn't be a problem, since it seems that you can access that media feature via JS too. Would probably be a rather simple modification to themeswap.js.