Yep, there has been a Dark theme in the To-Do app for a while now, so if you have not noticed before, we will tell you how to enable it and what it persists of. It has been improved in version 1.43.7617, released for beta users and here is the changelog for it:
We’ve continued to polish Dark theme support in our widget. With this update, we’ve added rounded corners and checkbox colors.
This update also includes accessibility improvements with several fixes for TalkBack support.