IMO this would have been the perfect way to ensure the feature won't be useful for anyone. You need buy-in from both developers and users for something like this. Developing one alternate color scheme that everyone supports is a thousand times easier to justify than developing n alternate color schemes that most people don't care about.

And by naming the function `light-dark()`, they explicitly left the door open to introduce something like you suggest in the future. Had they already named the function something more generic, it would limit the flexibility of any future API!