Color schemes (luminance modes) and color themes (aesthetic styles) are not the same thing. Halloween theme can have light and dark schemes.

Also, prefers-contrast media query exists, which can be used in combination with the luminance modes (color-scheme): halloween-light, halloween-dark, halloween-light-high-contrast, halloween-dark-high-contrast.

https://news.ycombinator.com/item?id=47832705

I’m not sure what kind of function you’re asking for that would support arbitrary user-defined themes. With the if() function and/or @function declarations you could build pretty much any system you want for defining theme-specific values for arbitrary themes.

It’s always irked me that light and dark are given privileged positions in the namespace. They’re just themes (or schemes if you like), like all the other possible themes.

Web standards are discussed in the open aren't they? I'll see if I can find what reasoning lead to this.