The question was how are you controlling the LCD.
- directly using Arduino pins
- using an i2c LCD backpack
or something else.
And then which LCD library.
You said LiquidCrystal 1.0.7, which comes bundled with the Arduino IDE but then you said you downloaded a LiquidCrystal library.
I'm not sure what you meant by that but the IDE bundled LiquidCrystal library controls the LCD with Arduino pins and doesn't not provide support for backlight circuit control.
If you are using Arduino pins and the IDE bundled LiquidCrystal library then you will not be able to control the backlight as that library does not have support for controlling a backlight circuit.
That library does have display() and noDisplay() but does not have backlight(), noBacklight(), on(), off() functions.
You could switch to my hd44780 library which would provide support for the other functions.
But you would also need to create a backlight circuit and would need to use an additional Arduino pin to be able to control it.
If you used a PWM pin you could even dim the backlight.
--- bill