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.
