I’m not sure if you really want to duplicate all of the CSS which defaults comes with Obsidian. I’d rather suggest locate the area you want to change, and make snippets for that area.
And even better, instead of getting to know all of the default styling, I’d rather focus on getting to know the Developers Tools screen, and how to locate elements within it, and finding it’s selectors, and which style is applied just now (aka Computed styles), and so on.
Learning to navigate Developers Tools efficiently, is way more useful than trying to pick apart all the various styling done for Obsidian in general. At least, that’s my view on this matter.