This article Explains in detail on how you can set up dark theme using Provider to manage state and shared_preferences to save theme preference. In this repository i have implemented dark theme in a most simple and easy way. Using this method you can implement as many themes as you want and specify your own color schemes as needed.

dependencies:
  shared_preferences: ^0.5.10
  provider: ^4.3.2+2

At the time I've implemented this dark theme these were the latest packages avaiable and most probably the newer versions will also work fine. Add a PR if anything gets depricated in the future and I'll merge it.