I'm aware of the prefers-color-scheme stuff added to browsers. AFAICT, only Firefox supports setting it at the browser level, while others will inherit from the system.

This would still remove the visitors ability to say "I want THIS SITE to be in light/dark mode", without affecting all other sites / apps on their system. This page could support reading from the browser/OS, while still giving the choice on top of that.

Again, this choice was probably implemented ... to give the user a choice.