[m-p-3]

One of my (maybe unwarranted) concern with automatic dark mode, is the potential ability of the browser and apps to determine my (very) coarse location based on when the dark mode is enabled or not, as it because another potentially identifying info among others.

Does that kind of CSS allows a script to determine if the browser is displaying dark mode or not, or send back that information to ads servers?

[teaspoon]

Yes, you could craft CSS to conditionally load an image based on the media query. This would let a server observe the client's setting.