Problem

From #20776.

The pane color for 3d plots (i.e. the background planes limiting the 3d view box) is hard coded. It would be reasonable to make them user-adjustable.

Proposed solution

Possibly an rcParam would be enough (no need for an explicit setter).

Complication: The color values are hard-coded in mplot3d.axis3d.Axis._AXINFO. They are shaded slightly different for the three planes to create a better 3d effect. We'd probably need to expose all three colors and explain how they are used.