How is LED backlight brightness adjusted?
You are right. When driving LED with continuous current, brightness depends on the current. But in the era of popularity of cheap microcontrollers it becomes easier to control LEDs with PWM -Pulse Width Modulation. Rough explanation: Diode is powered for very short time, then is off for short time lets say 500 times a second. The brightness depends on how much time the LED is on vs how much time it is off.