Application.AllowDarkMode

From Xojo Documentation

Design Property (as Boolean)
This Application property can only be set in the Inspector and is not available in code.

You can only set this property in the Inspector for the App object. Set to ON to make the app run in dark mode when dark mode is set in the OS settings. This is set to False by default.

Notes

Dark mode is only relevant on macOS Mojave (10.14) or later.