This is because choosing dark mode on your mobile means you have a preference for apps (or sites) which are fully compatible with dark mode. Dark mode on my Discourse site is a choice I give to registered users (they can choose it in their settings), after they are registered and logged with the default theme, me, the admin has chosen. As you know not every app is taking into account the operating system dark mode. I would like people browsing my site are presented with what me the admin has chosen, then, and only then, what they have chosen, not the other other way around. Who is the boss? 