Darkmode and its future?

I think darkmode will be the future so I‘m wondering where to take care about integration.

1. Is it smth which a website developer should implement fully?
2. Or is it maybe more of a „switch“ logic within a website, which tells the website how to be shown in a dark/light environment.? So it could be just switched with a JS oneliner? Like https://darkmodejs.learn.uno/
3. Its the task of a browser? They do support these two modes:
   prefers-color-scheme: dark

Battery safe time is also unexpected good!