Dark mode for Verso HTML output (prefers-color-scheme)

[sirbuffalo]

The Verso works I've used (e.g. Functional Programming in Lean) don't provide any options to be read in dark mode. This makes the experience of tabbing back-and-forth between apps that support dark mode (VS Code) and Verso-generated material quite jarring on the eyes.

It would be great if Verso works automatically supported OS defaults for dark mode. Perhaps by default, the CSS Verso generates could include a @media (prefers-color-scheme: dark) { } set of theme overrides?

If not, what can Verso do to at least encourage (and make it simple for) works to support dark mode? Possibly related: Issue #216

[david-christiansen]

I would like to implement this, but right now it's lower priority than a number of other tasks.

A PR would definitely be welcome, though! Ideally it would harmonize colors (both light and dark) with those used on lean-lang.org.