With Android 10, Google finally added a system-wide dark mode to its OS, but unless you want to rely on a third-party tool, there is no way to automatically schedule a theme switch on stock Android. It looks like that's bound to change, as Google wants to introduce that capability with "a future Android release," presumably version 11 of the OS.
In the Google Issue Tracker, a problem called "Scheduling Night Mode on Android Q final release" has been marked as fixed by a Googler who added, "The issue reported here has been fixed and it will become available in a future Android release." Since Google just invented Pixel feature drops, it's entirely possible we might see this functionality in the near future if it considers these as future releases, but it might also take until Android 11.
After testing and subsequently removing an automatic switcher in a beta build of Android 10, Google started to argue against it. The company says that upon changing themes, applications have to redraw their interfaces, meaning you'll lose your scrolling position or even entered text in some cases. Reliably detecting sunrise and sundown is also difficult.
I've encountered this problem way too often thanks to Android 10's power-saving feature that turns on dark mode automatically at a set battery percentage. Hopefully, Google managed to fix the issues, so switching themes won't be as jarring as it is today.
- Google Issue Tracker
- XDA Developers