To know how to install the screensaver in your Panel PC open the following link:
Install Screensaver on your Panel PC
In order to eliminate the backlight in the screensaver mode you must follow the following steps:
1) Open the Touchberry Pi
2) Make sure there is Internet connection
3) Edit the file: /boot/config.txt adding the following line:
hdmi_blanking = 1
4) Edit the file: ~/.config/lxsession/LXDE-pi/autostart and add the following lines:
@xset s 0 0 @xset s noblank @xset s noexpose @xset dpms 0 0 0
5) Now, reset your Touchberry Pi.
After following these steps you have already turned off the backlight in the state of screensaver.