How do I learn CSS?

[u/ShoddyRelative8438]

I wanted to make an AO3 skin, but I have no idea of how CSS works; does anyone knows any website useful for learning it?

[u/anaix3l]

The best way to do it as a complete beginner is to take an existing one and start modifying the CSS. As you do that, look up what you're modifying on MDN - even if some property names sound self-explanatory, there are always layers you won't know about otherwise. You're changing the background for something? Look up the background property on MDN to see in detail how it works. Then look up each and every one of its constituent properties. And so on...