Support for dark mode

[david-christiansen]

CSS now has the ability to indicate whether the user has request light or dark mode: https://developer.mozilla.org/en-US/docs/Web/CSS/@media/prefers-color-scheme

It would be great if the generated site supported this!

[Abastro]

Was just going to suggest this exactly, I am looking forward to it. Thanks for great work!

[jzwood]

@david-christiansen PR for dark mode: #383

[jzwood]

@david-christiansen Since themed hljs is captured here, #385, I think we can close this.