How to Get Started With Custom CSS?

I have a decent knowledge of CSS and I want to get started making my own obsidian theme.
But I have no idea how to, I can find no source that actually lists everything I can do and I don’t think scraping things I saw on different sources is the best way. So what is the better
way? is there official documentation for the custom CSS stuff?

No official documentation, sorry. There are quite a lot of options, and they change and develop often as Obsidian is still technically in beta. In other words, documenting them would be a non-trivial task!

Search the forum, however. There have been a few community-led attempts at getting started guides.

Otherwise, you can use Chromium’s dev tools (built into Obsidian) to explore the DOM/CSS on your vault.

ok, thanks for the informition