When using the Spy skin, don't put a name. Let yourself be "An unnamed cell."
Use the dark theme. The dark theme will just make you visible to yourself. It doesn't matter if you use the dark theme or the light theme, but if the other players also use the dark theme, you are completely visible to them.
This skin very likely was inspired by Agario Youtubers who originally went on Agariomods and made their own custom skins that looked like the background of the map to troll players, just like this one.
This skin was the most popular skin in December 2015-February 2016
Players who are split cannot see the Spy skin as easily as others, as their screen is zoomed out more and causes the skin's outlines to disappear.
The outline of the Spy skin will be visible.
People rarely use this skin now, because of many having extensions with their own skins.