I think you misunderstand how this works. The OS doesn't magically "provide" dark mode. It merely offers a system-level settings switch. It's still up to the app itself to implement dark-themed styling it can switch to when the OS setting changes. This is exactly how it works on Android, for instance. Not to mention there are plenty of Windows apps that enable dark mode already - you really don't need a system-wide setting for that. I've got several of such apps open at this very moment: Notepad++, Sublime Text, Android Studio, Skype (the classic one, not the UWP version), Fork.