I am using a 2010 iMac as a second monitor for my Mac Pro 5,1 in Sierra. I have it working with sound but the brightness controls on the iMac keyboard (apple bluetooth) are not working. I did a lot of searching and saw that this has been an issue in some versions of OSX but I can't seem to find a clear fix, if there is one. If anyone has an answer that would be great! Thanks guys!