I use an iPhone X and have paid for the dark mode feature in beorg. I would love to see a true black mode to take advantage of the benefits of the OLED screen. I would imagine this wouldn't be too hard, maybe changing a couple of the colors in the scheme and keeping the rest fairly similar. I love beorg and use it alongside emacs and org mode daily for my task management!
beorg will eventually support the iOS 13 dark mode features which will replace the need for the dark mode extension in beorg.
You do have however almost complete control over the colour scheme used in the beorg UI - with a little work. Check out https://beorg.app/learning/custom-colors/ for a guide on creating your own custom colour scheme. If you do create something worth sharing feel free to add to the forum.