This plugin utilizes LethalLevelLoader to add unique skyboxes to moons based on their theme (content tags). It is configurable and has a user blacklist for individual moons.
This mod is fully client-sided, and there is fallback support for vanilla if LethalLevelLoader is not installed.
The Amethyst skybox also has special fog that applies to interiors. (configurable)
Screenshots
Poly Haven - HDRI sources
IAmBatby - LethalLevelLoader's cool tag system