MacBook Pro backlight is dead but I can faintly see the screen

Shine a torch at the screen at an angle - if you can make out the desktop, the panel's fine and it's only the light behind it that's gone. On the 2016-2017 models that was usually the display ribbon cable wearing through, and it tends to start with the light cutting out once the lid opens past a certain angle. Does it come back with the lid half shut? Either way it's a repair, not a setting. An external monitor keeps you going in the meantime.