Like I said in the first post, tvOS and OS X have a dark mode but all they do is darken the menu, menu bar and dock; not any apps are darkened whatsoever. So there are no elements in iOS that would turn dark.
All apps are designed the way the developers want it, turning all the white space into black would look horrendous with some colors, and recoloring elements/buttons would just confuse people. They would also need to have dark mode in mind when designing their app. Even if a jailbreak could do it, it's always up to the developers to implement it. Which I doubt it's worth the effort to redesign their apps, so adoption would be slow- which means your app switcher would look like mayhem.
I also like to say, that I find "dark mode" worthless. If you leave an app that has dark mode, you will still be shunned with your bright wallpaper in the app switcher or your home screen. Not only that, but the icons too, let alone apps that don't have dark mode, or have images displayed on screen (i.e. Instagram, Facebook pics, photos app, Imgur pics, and most of them are edge to edge pics, so they take up most of your screen real estate.)