On the website:
From your Planner page, select the gear icon to expand the Settings menu. The bottom Theme option will allow you to switch between Light and Dark views.
On the app:
On the mobile apps, the display will automatically be set to light or dark mode based on your device settings. See how to adjust this according to your OS in the articles below.
Use Dark Mode on your iPhone and iPad
Change to dark or color mode on your Android device