This effect causes filled rings to be outlined (i.e. drawn as lines), using the background color. Rings that aren't filled are unaffected.
Generally, filled rings tend to look gaseous without outlining, and faceted with it. Note that without outlining, neighboring filled rings of the same color are indistinguishable, though this isn't necessarily a problem.
To enable/disable outlining for new rings only, use Image/Outline or F7. Since existing rings are unaffected, the change occurs gradually. To enable/disable outlining for both new and existing rings, use Image/Invert/Outline or Shift+F7.
Outline increases graphics overhead and may reduce performance.