Modding Help How do I change parallax pic for a planet's underground area?

I'm currently working on a large building project that will have underground constructions. The problem is, the planet where I build was originally a barren planet, until I terraformed it into a scorched one for its features. Therefore its parallax pic underground stayed the same, and never gets to change, no matter how deep I go. I think it's because terraforming can't change anything underground?
I've read some old threads, they did mentioned that I'll need to edit game files, but I can't figure out how to do it. I only ended up not finding any folder named "parallax".
So, how exactly do I change parallax pic for a certain planet? If it doesn't work that way, maybe I'll need to change default parallax for that certain underground biome?
Also, is there actually no mod that can help me with this ;_;