Ok, what type of device do you have? It is really difficult to know who to fix the brightness if you do not provide us with the type of device. Second, sometimes if you touch the top and slide down there is a brightness thing that you can move.
Honestly, the only way you can find out what is the cause of it would be to open it up and determine if it is caused by the spilled coffee. It sounds like liquid damage but again we need to open it up to determine what it is.
it could be long screw damage or it could be the frontal camera. I check that out first though to see if there is any structural damage to the logic board