Is a LCD backlight adjustable?
LCD backlights are simple LEDs and it depends on the device if it is adjustable or not. In most cases it is, and it is usually controlled by pulse-width modulation (PWM). Which means that the supply voltage of the LEDs is applied discontinuously, and the longer these pulses the brighter will be the LEDs. 



This happens rapidly so for our eyes it seems continuous in every cases, we only see the LEDs less bright when the pulses are shorter.

The LCD's content is visible and readable without backlight, it only requires ambient light instead.