To change the backlight setting:
Slide the programming flap lever on the right side into the RUN position.
Press and hold both the tick (OK) and - buttons together for roughly 8 seconds until the screen briefly flashes "SET UP MENU" followed by "SET INSTALLER OK".
Press the + button to navigate past the clock format parameter to the BACKLIGHT setting.
Use the + or - buttons next to the time display to change the backlight parameter:
Press the tick (OK) button to save your change.
Slide the lever to another position and then back to RUN to exit installer mode.