Try holding + and - while turning on with longer than normal press of the on button. Then + and - will change intensity of back light. Not tried it but that is the process according to my manual Page 3 bottom right. Hope that helps
Just tried it and it works. Took me a couple of tries to get the correct medium duration hold of the on/off button and then it goes into a display mode, showing versions of hardware/software etc. Whilst it is doing this the + and - keys alter the backlight settings.
