What do you want to learn? It’s hard to point you in the right direction if you don’t have a direction. Yes, you’ve told us “HTML and CSS”, but that has nothing to do with Atom. You can start writing HTML and CSS with pen and paper, if you wanna be oldschool about it. What to learn about when learning about Atom will differ greatly based on what features you want in an editor, because Atom’s features are inherently malleable. For instance, if you want a live preview of your code, you would do well to download
browser-plus and then learn about that, but for someone using Python for data science,
browser-plus might be no more than a way to keep docs pages open in Atom while coding. If you plan to use SSH for remote work,
atom-commander is great. If you want to add buttons to Atom’s GUI, I can recommend
flex-tool-bar, and so on.
Since you’ve read the Flight Manual already, I don’t need to tell you about the Command Palette or how to write snippets.