The Dark Mode theme is available in iOS 13 and newer. In this app you have 3 options:
applies the dark theme to the app even if iOS is set to the light theme
applies the light theme to the app even if iOS is set to the dark theme
follows the theme set by iOS (default setting)
To update theme used by the app:
Access the Settings page
Scroll down to the My Style section and tap the "Dark Mode" row.
A list of dark mode options will load, you then tap the row with the theme you would prefer.