These appearances can be set manually in settings, or users can choose to automatically switch between them based on the brightness of their iOS display. Things 3.7 for macOS gained support for Dark Mode this past September.
Things allows users to organize their goals with task management features that include daily breakdowns of incoming tasks to finish, as well as projects like getting ready for a vacation. Things 3 integrates with Apple's Calendar, Siri, Reminders, and the Today Widget, and supports Handoff, Notifications, and Apple Watch.
Things 3 can be downloaded from the iOS App Store for $9.99 [Direct Link].