Okay so I figured out the way to get the secret skins is completely random when you load the game.
SO FAR. I've found 4 different skins:
1. Default
2. Purple
3. Red
4. Retro
(Specifically not naming them to avoid spoiling it for others)
I refreshed about 50 times and saw all of those once with the exception of the purple skin which I saw like 3 times... I wonder if there's more.