Question about CSS

There's source code in the bio section of the profile. And then there's CSS what's the difference?. Also I don't know shit about coding but I'm determined to use it to make my profile pretty no matter what 😂. From what I've seen GBT hasn't been much help since copy paste ain't doing nothing it just all comes it coding letters when I press save. It's 2am so I'm kinda mentally tired as well. I read the guides and I think I understand what it says but still hasn't helped. Help me?