A screen brightness changing fix for Retina MacBook Pros that worked for me.
Download: Here
My issue started when I replaced the whole upper lid of my Retina MacBook Pro Mid 2014. I noticed that I couldn't change the brightness of my screen, so I did a long research seeking for an answer.
I've made a program that's compatible with all the versions of macOS. Basically what it does is resetting your screen completely.
Just open it, and it will do it's thing.
You'll notice that it will turn off and back on. After that, your screen brightness should change properly when you adjust it.
This fix in order to work, it should be opened at every boot. So put it in the login items in your macOS "User" settings.