Install cairo-dock-xgamma-plug-in by entering the following commands in the terminal:
sudo apt update sudo apt install cairo-dock-xgamma-plug-in
Xgamma plug-in for Cairo-dock
A collection of official plug-ins and applets for cairo-dock. . This plug-in setup the luminosity of your screen directly from your dock. User can also define a luminosity value that will be applied automatically on startup.