I have one of this shields, and notice that pin 10 (not documented in the manual) is the backlight led controll, so if you send diferent values of PWM to that pin you can dim the backlight.
Althought I got a problem, if not sending anything to that the white dots from the display have normal intensity, but has soon you play with pin 10 PWM even sending 255 I never managed to got the same intensity.
Anyone face this problem or know a work around to get back the maximum intensity?