And as I and many others have pointed out, this isn't even remotely close to accurate description of what would go in to a hypothetical Dark Mode. This is a prime example of the amateur hour that I'm losing interest in here.
Er, can you elaborate further? Perhaps you're referring to support for third party apps?
Let me run with just one out of a million solutions to this.
The dark mode setting value would be readable by third party apps.
By default, all apps would be marked 'not ready for dark mode' and would continue as before, regardless of this setting, until the developer takes steps to say it's ready for dark mode and submit and update to their app.
For some apps using very standard UIs, it might be a case of the developers flipping a switch to say 'ready for dark mode'. This might enable dark mode versions of standard UI elements in their app. The developers would have to review how this minimal intervention panned out in their app, and make any necessary adjustments. This is directly analogous to how 4.7" and 5.5" modes worked. Many apps had already been designed with auto layout, and simply flipping the switch to enable native support for larger screens meant their apps worked instantly. However, many probably had to make a few tweaks, and many chose to redesign their app to make more intelligent use of the extra space. This would be the case with dark mode too. Some apps with very custom UIs would have to put a lot more work into it, but it's nothing unachievable. Some apps might already be working in what's effectively a dark mode- for them, it might be a case of actually designing a light mode for use when dark mode is off. Some apps might already have their own app-level dark/light setting, and they might decide to remove their app-level switch and make their app respect the system level setting instead.
Only when the app developers are happy with the results would they say their app is ready for dark mode, and ready to respect the system setting, and then submit their app.
Apple would provide documentation, guidelines and support for all developers developing simulataneous light and dark modes.
Apple would have to add an extra review step for apps submitted to the App Store, to ensure those making use of the dark mode setting respect it in a reasonable manner.
Windows Phone already has a dark/light mode which third party apps can take advantage of.
What else is there?