The other issue is many sites don't use that CSS media query at the minimum for auto setting the theme. They instead use a Javascript approach that often involves local storage/cookies even if no choice is made, which doesn't work if those are blocked and/or Javascript disabled. In such cases the default theme is forced.

The optimal approach is applying the appropriate `prefers-color-scheme` using CSS alone, while additionally allowing a theme override using JS/storage. Fewer do this though, even though it wouldn't require any cookies and thus no consent dialog.

The worst are sites that only have theme switching gated behind registration.