git clone https://github.com/JonMagon/plasma-screendimmer.git
kpackagetool6 --install package
Right-click the task bar, then click Add widgets, search for Display Brightness and add to the panel. You can move it if necessary.
Alternatively, right click on the system tray, open settings, in entries list search for Display Brightness and enable it.
- Modified BrightnessItem.qml owned by KDE Team