Thanks for playing!
I stole the idea from Sebastian Lague from this video (link with timecode), but simplify it a lot.
The only things I changed - instead of "depth map" I just used radial gradient from the island center and colorize itas gradient between 3 colors and I didn't add normal maps. And I did remove glare, cuz it was looked strange in my toon shaded game.
I also tried to recreate this shader from roystan, but didn't like the result.