Add support for variants (light & dark mode)

[helje5]

I think this can also be done using just plain CSS (switching the site on light or dark mode).

A small JS button to let the user switch shouldn't be hard either?

[Tyler-Keith-Thompson]

FWIW there's also a media query that reads from OS settings: https://developer.mozilla.org/en-US/docs/Web/CSS/@media/prefers-color-scheme

That could be a replacement for a button, or at least a default value for said button.