Just got done making some icons for all of my outfit toggles. It was probably too much work for something only I am ever going to see but I thought I would share anyway. Using this combination of solidify and smooth modifiers in blender I got this nice outline effect. The first solidify is just cleanup to make sure the mesh is solid for the outline, the second solidify is where the magic happens. Every object has 2 materials: black and an unused white emission material with backface culling. "Flip Normals" and "Material Index Offset" on the Solidify.001 modifier is what accomplishes the outline effect.