In the Control Panel, the Horizontal/Vertical and User-defined texture shading modes have two new sliders in their option section: Ambient Light and Diffuse Light.

The Ambient Light is an illumination that comes from all directions and lights all objects uniformly. The slider range is 0 (no illumination) to 1.

The Diffuse Light indicates the ability of the objects to reflect light in all directions. The slider range is 0 (no light is reflected) to 1.