Feature request: Auto switch dark/light mode themes based on OS

It would be nice to be able to set two themes, one for the OS light mode and one of OS dark mode and autoswitch based on OS state. Something like:

```yml
theme-dark:
background-color: 240 21 15
contrast-multiplier: 1.2
primary-color: 217 92 83
positive-color: 115 54 76

theme-light:
background-color: 0 0 95
primary-color: 0 0 10
negative-color: 0 90 50
```

Using CSS like this:

```css
@media (prefers-color-scheme: dark) {
...
}
```