Respect OS color scheme for automatic dark mode

[xixixao]

Feature suggestion: dynamic dark mode.

I'd suggest:

Allow devs to pick how dark mode is supported:

1. Fixed (current, default)
2. OS preferred (via prefers-color-scheme, easy to implement)
3. Manual switcher (can be two way or three way, user can switch - I think it's overkill over 2.)

Ideally I'd just have something like:

{
  "styles": {
    "dark-mode": "prefers-color-scheme",
    "theme": "axo"
  }
}

Thank you and congrats on this awesome project!