Great, with Version 1.3.0, Ly display manager introduced a brightness feature, where you can increase or decrease brightness. I never felt the urge to change the brightness of my black login screen with green text, but now it's there.... Pressed F5, nothing happened but a message appeared "failed to change brightness".
It seems that Ly tries to use brightnessctl to adjust display brightness, which is not available on FreeBSD.
I really liked Ly because it's a simple and easy to use display manager with a minimalistic approach, but I have the feeling that more and more open source software is being developed with a focus on Linux, leaving other systems out in the cold.