Hi MatildeRizzi,
I'm sorry that previous solution didn't worked. Could you explain me in detail which output you receive when you press F3 or Fn+F3?
According to your keyboard, usually it should control monitor Brightness. Is It working fine outside Minecraft program?
Can you eventually go to options menu in Minecraft? Press “Esc” which is on your keyboard. That should open up the menu/pause screen. On that menu, you should find a button labeled “Options…”.