Listens for keyboard brightness keypresses and alters brightness on all supported displays.
Note: This is primarily intended for Macs with no internal display, but should work for those with an internal display.
Note: External displays will only respond to brightness change requests if they were either made by Apple, or are LG UltraFine displays (which were designed by Apple).
Note: If your keyboard brightness keys aren't triggering this Spoon, use Karabiner Elements to set them to display_brightness_decrement and display_brightness_increment.
Download: https://github.com/Hammerspoon/Spoons/raw/master/Spoons/AllBrightness.spoon.zip