Or do you mean we should respect an explicitly set browser preference for “dark”? I don’t actually know whether that’s possible while also not following the “auto” setting.
Indeed this is not possible, since there is not actually such a thing as an “auto” setting as far as CSS is concerned, but rather simply a system where the prefers-color-scheme media query matches the light value by default, but will instead match the dark value if the browser is commanded to make it so (either manually by the user or automatically by the system). An “auto” setting exists on the system level, and can exist on the website level (as on gwern.net), but not on the “intermediate” level of CSS and browser implementation thereof.
Indeed this is not possible, since there is not actually such a thing as an “auto” setting as far as CSS is concerned, but rather simply a system where the
prefers-color-schememedia query matches thelightvalue by default, but will instead match thedarkvalue if the browser is commanded to make it so (either manually by the user or automatically by the system). An “auto” setting exists on the system level, and can exist on the website level (as on gwern.net), but not on the “intermediate” level of CSS and browser implementation thereof.