Outline
Add an outline on all objects of the layer having the effect. This won't work well if shown on top of the background color of the scene. Be sure to use an object acting as a background or a floor.

Use the Thickness property to set how wide the outline is, and Color of the outline to set its color.
Note
This effect only works on 2D layers and objects.
Reference
All effects are listed in the effects reference page.