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.

Properties
Use Thickness to set the width of the outline and Color of the outline to set its color.
Note
If a thick outline looks cut off at the edges of the object, increase the Padding property to enlarge the area in which the effect is drawn.
Reference
All effects are listed in the effects reference page.