How would one make and install a custom sky?

Hey everyone, I was wondering if custom skies where a thing since basically all the sky is in the Sims 4 is just an image for effects and ambiance. I have found a bunch of cool and fun Medieval Custom Content and I was thinking about making a separate Sims 4 folder where I can make and play in a purely medieval themed world. I have already begun building medieval structures and stuff, but one thing I noticed is though I can make some amazing medieval style builds, the sky in some of the maps kind of give away the feeling. Many of the sky backdrops in Sims 4 have large cities and stuff showing in them. Which are not really a problem as I can ignore them, but it would be cool if I could change them to a medieval theme like maybe a stary night sky or a large forest. Thank you in advanced.