Not a bug, but similar

So im trying to make a minecraft texture pack for my friends and what im trying to do is change the screen to load in a world or go between diemensions. But when i went to the minecraft textures folder i didn't see anything resembling the backrounds screens for them. I have looked it up and i have found nothing so i made this post. Just wanna know if it is possible to change it or if im missing something. Thank you.