If you're sporting the Microsoft To-Do app on Android, and really want a dark theme, you might want to join the Microsoft To-Do Android Insiders group and download the beta version of the app today. The latest version 1.37.7599 update introduces this feature, and other fixes as well.
— Microsoft To Do (@MicrosoftToDo) August 6, 2018
To find this latest beta dark-mode option, just tap your profile icon on the top left, and then click where it says Theme. You can then choose the dark theme, and allow the app to refresh itself with the new colors.
Other changes in this beta version include the ability to select theme color when creating or editing a list. Ther's also fixes for a recurrence behavior, and for a crash on tablets after completing a task.
If you want to join the Beta, all you need to do is head to the Google Play Store listing for Microsoft To-Do and scroll to the bottom and opt-in. You can also sign up here to get Google+ notifications about Microsoft To-Do updates. Everyone else will be seeing these features soon once beta testing is complete.