This sounds like it would have the same problem as redirects to mobile subdomains - users will share links to light. and dark. subdomains, unknowingly forcing their preference onto the recipient.
The proper solution is to only implement the media queries and yell at browser vendors to make this easier to toggle either for all websites or for specific websites. Implementing a light/dark mode switch in every website is absurd.
unknowingly forcing their preference onto the recipient
I'd suggest knowingly rather than unknowingly. It's a link to a website after all. It should link to the same thing that the person who shared it was viewing. That's the point of a URL - when you share one the recipient should see what you were seeing.
FWIW I would also like browser vendors to do a better job at enabling users to manage their preferences on a per site basis though.
The proper solution is to only implement the media queries and yell at browser vendors to make this easier to toggle either for all websites or for specific websites. Implementing a light/dark mode switch in every website is absurd.