Tap “More” in the bottom right corner of your screen
Under the section titled “Theme,” choose between System, Light, or Dark
"Dark" and "Light" modes should be self-explanatory. "System" mode will put the app in light mode when your phone is in light mode, and dark mode when your phone is in dark mode.
Now that you understand how to switch between dark and light mode, you might enjoy one of these articles next:
How to Customize your Dashboard
How to Configure Your Food Logger
How to Configure your Shortcuts and Toolbar