View > Display > Gradient Background
Turns a gradient background on and off in the Main view. A gradient background also applies to the model as it displays in the Renderer window. By default, the gradient background is turned on. Select View > Display > Enivornment Settings > Gradient Background to define the top and bottom horizontal gradient colors for the required effect. For more information, see Create a Gradient Background.
Select View > Display > Gradient Background.
Click Activate Gradiant Background from the Views toolbar.