Hold , and select > System > Backlight.

Mode

Sets the backlight to turn on when you interact with the device, which includes receiving a notification or using the buttons or touchscreen.

Brightness

Sets the brightness level of the backlight.

Timeout

Sets the length of time before the backlight turns off.

Gesture

Sets the backlight to turn on when you rotate your wrist toward your body to view the device. You can use the Only During Activity option to use this feature only during timed activities. You can also adjust the gesture sensitivity to turn on the backlight more or less often.