Hold the touchscreen, and select Settings > System > Backlight.

Mode

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

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.

Timeout

Sets the length of time before the backlight turns off.

Brightness

Sets the brightness level of the backlight. During an activity, the backlight uses the brightest setting.