Well, I’m really still a beginner and hadn’t touched css before December. Like I said above, I was only able to make the banners with @efemkay’s code.
If you want to get started learning css though, I’d recommend you start experimenting by tweaking existing themes’ code and looking at other people’s css. I started by messing with the Mado Miniflow theme. The result of my tweaking was an abomination, but it gave me a feel for how css works. After that, and a few hours of pasting css from Wikipedia, I’ve been able to make simple themes from scratch.
Sololearn has a free course on css that you can also view if you like. However, I think the fastest way to learn is by observing, replicating other people’s css tricks, and experimenting.
I’d also recommend reading these posts. They’re a great introduction to making css for Obsidian.
Getting comfortable with Obsidian CSS
Common Selectors for Custom CSS