You can specify the following settings when your 3D model is set to use a perspective
view. These settings are available in the Options dialog box, Display tab, Colors
option:

• Ground Plane. When perspective projection is turned on, the XY plane of the UCS is displayed as
  a ground plane with a gradient color. The ground plane displays a gradient from the ground horizon to the ground origin.
• Sky. The area not covered by the ground plane is the sky, which displays a gradient color from the sky horizon to the sky zenith.
• Underground. If the ground plane is viewed from below ground, the ground plane displays a gradient
  from the earth horizon to the earth azimuth.
• Ground Plane Grid. When perspective projection is turned on, the grid is displayed as a ground plane grid. Colors are set for major grid lines, minor grid lines, and axis lines.