At the ongoing Android Dev Summit, Google talked about how Dark Mode can have a positive impact on battery life, especially when it comes to OLED screens. Google happened to look at various parameters that affect power draw such as brightness and color. Hopefully, this means future Android versions will have an option for a native OS-level Dark Mode.
https://www.notebookcheck.net/Google-finally-realizes-that-Dark-Mode-is-good-for-the-battery.358156.0.html