The Brightness Controller should be launchable with the app-finder (Zorin OS Lite) with
Or can be launched the same way from terminal.
Opening the App Menu, I started typing "brightn" into it, and it searched for and located it.
If neither of these are working, please try:
sudo apt install --reinstall brightness-controller
I never installed this app before, in spite of recommending it a lot to others. For the first time, I installed it in order to ensure I was giving you accurate information... And it Works Like A Charm.
I use qt5ct which makes it use my custom gtk theme.
You can also add it to the Panel (Zorin OS Lite) by adding a Launcher, then editing its settings and scrolling to Brightness Controller.