The brightness is not supported in the simulator. You will need to try it on the micro:bit itself!
Set Brightness
Make the LED screen as bright as you say when it is turned on.
led.setBrightness(121)
Parameters
valueis a number that means how bright the screen is when it is turned on, from0(darkest) to255(brightest). For example, the number127means the screen is halfway bright when it is turned on.
Example: change brightness
This program makes the screen brightness 100% (255). Then it turns on
the center LED (2, 2), waits for one second, and then sets the screen
brightness to 50% (128):
led.setBrightness(255)
led.plot(2, 2)
basic.pause(1000)
led.setBrightness(led.brightness() / 2)