I am struggling for a picture of a “light” (216) rejoicing. It is a common word and the first place we find it being used in the Bible is,
(Gen 1:3 KJV) And God said, Let there be light: and there was light.Light was the first act of creation, produced by the fiat of God. I would guess that it is necessary for any kind of celebration. I have not heard to too much joy being expressed in total darkness. Darkness is usually associated with fear and weariness.
Light allows us to see and that gives us access to so much we can enjoy. In the context here it would seem like that light is what we would call back-lighting. My current Kindle has this feature. The light for the page comes from inside the devise. It makes it possible for me to read in a dark room or airplane.
So? God puts the source of joy in our hearts. He does not hang it around us but generates it from inside. It changes the way we perceive our environment. Rejoice.