Install xfce4-power-manager-plugins by entering the following commands in the terminal:
sudo apt update sudo apt install xfce4-power-manager-plugins
Description:
power manager plugins for Xfce panel
This power manager for the Xfce desktop enables laptop users to set up a power profile for two different modes "on battery power" and "on ac power" while still allowing desktop users to at least change the DPMS settings and CPU frequency using the settings dialogue.. . This package contains the brightness plugin.
Homepage: http://goodies.xfce.org/projects/applications/xfce4-power-manager
Version: 1.6.1-0ubuntu1
Section: universe/xfce