git clone https://github.com/JonMagon/plasma-screendimmer.git
cd package
kpackagetool5 --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.
- BrightnessItem.qml owned by KDE Team