Ok, I was wrong, apparently for I2C type LCD, the brightness and dimming values don’t matter:
https://github.com/OpenSprinkler/OpenSprinkler-Firmware/blob/master/OpenSprinkler.cpp#L1631
This can be improved in the next firmware update.
The backlight does come on if you click any button, and turn back off after 15 seconds of inactivity.