I think I'd just separate the two things:

``` Theme ( ) Light (•) Dark

[x] Follow system Dark — currently controlled by system ```

When "Follow system" is on, the theme choice is greyed out.

Feels simpler to me, and it scales better if themes ever become more than just light/dark.

I used the Selenized color scheme once and it indeed came in four variants: light, dark, white and black.

One may also think about a grayscale theme, perhaps.

Indeed. Or shorter: “auto”