Press Menu, and select Settings > System > Backlight.

Toggle switch

Allows you to turn the backlight on for interactions. Interactions include the buttons, touchscreen, and notifications.

Gesture

Sets the backlight to turn on for wrist gestures. Gestures include rotating your wrist toward your body to view the device. You can use the Only During Activity option to use gestures only during timed activities.

Timeout

Sets the length of time before the backlight turns off.

Brightness

Sets the brightness level of the backlight.