Perhaps CSS ( Cascading Style Sheets ) could be learned in a single day if one had a tutor guiding their every step and misstep for a couple of hours. But, without that help it doesn’t seem possible. There are simply too many loose ends to explore with trial and error explorations. These somewhat random wanderings take time and patience.
Online CSS tutorials
There are online tutorials that claim show the way, and probably they do, but it requires actually doing the operations several times before the logic of the system becomes second nature. The online tutorial I did watch jumped around like a nervous jack rabbit. He nibbled here and there and probably chewed up most of the key issues, but what is needed is a solid wedge into the system which is simple enough to learn and which works reliably. The avalanche of beautiful options which CSS provides can be learned later, but until one has a firm grasp of a successful entry point the rest of the beautiful stuff turns into a blizzard of snowy white blindness. The experts claim CSS is easy. Here is a CSS tutorial that is about as clear as they get.
If something is easy, it should be easy to teach, and easy to learn.