I'm building an city, but i have background with mountains and stuff. It's on an barren planet. Nah nah, don't send me Citybound. That is used for lush planets so i wouldn't use that, of course i would not start over because of an background. I wonder how could i make a mod or just change it to make a city-like background using code. Any advice ??
Using code? You mean like a admin command? Cus that isn't possible. Otherwise, you need to be familiar with planet biome configs and their associated parallax configs.
How familiar are you with JSON? I suggest you start here: http://community.playstarbound.com/resources/unofficial-modding-ebook-2-0.2930/