"backlight-gui" is a tiny GUI utility to control the display brightness via /sys/class/backlight/.
You can download the software packages arranged for installing on Raspberry Pi OS from the Releases page.
Note: This software is currently tested on Esrille Unbrick series.
To build and install "backlight-gui" manually, run the following commands:
git clone https://github.com/esrille/backlight-gui.git
./autogen.sh
make
sudo make install