Using what did VKontakte make a virtual flashlight for Earth Day 2015?

Question

css

Half an hour ago, an offer to join the action appeared on the main VKontakte (photos will be below). And since I'm a web developer, I immediately wanted to know how they did it? What technologies were used? What can be used to determine this, how to find the piece of code that does all this? Can the inspector do this if you know that everything is tied to the mouse event?
A photo:

Answer 1

background: -webkit-radial-gradient(200px 200px, circle contain, rgba(0, 0, 0, 0), rgba(29, 29, 29, 0.96)) center center;