Robots and entities should take on underlying cell background unless overridden

[kostmo]

Currently, any entity or robot with an unspecified background attribute gets a black background by default.

A potential advantage to the current behavior, at least for robots, is better visibility contrast. However, it arguably "breaks immersion" and is less aesthetically pleasing than taking on the underlying background. And anyway it shall still be possible to explicitly set the robot's background color as a custom attr, if desired in some scenario.

[byorgey]

In principle I agree with this idea, although it may require changing some color schemes. There are probably some current entity/terrain color combinations that would be difficult to see.