The ambient light sensor on the dashboard should automatically change the system between Day and Night mode, but this is probably not what you're talking about.
There is also a brightness & contrast control. From the home screen, touch the gear icon at the lower right, then General. Touch the Display button at the top. The controls are at the bottom left of that screen. The inner left/right buttons change the current setting and the outer left/right buttons switch between Brightness and Contrast. The adjustment range of brightness did not seem that big to me.