Some of the documentation is better than others, but yes, there are some assumptions made in a lot of it. For example, you mention:
I don't know how much you know about web development (Atom is built on top of Chromium, the open source base of Google's Chrome), so I'm going to start with the basics. Unfortunately, it is impossible to list the set of all possible CSS selectors. There are some common ones, like the ones I mention above ... but it quickly gets out of hand, especially because any package with UI can introduce new elements, classes, attributes and the like that increase the size of the selector domain exponentially. This article was a great help to me in learning about CSS selectors:
The selectors I mention above,
atom-text-editor, are elements.
You can open the developer tools, just like you would in Chrome, by going to the menu
View > Developer > Toggle Developer Tools. This will allow you to inspect the HTML, CSS and etc that is used to build the UI of the application live. From there, you can decide what CSS selectors to use for various parts of the UI. Also, if you open Atom in Developer Mode by executing
atom --dev from the command line, you can right-click on any part of the UI and select
Inspect Element from the context menu that comes up. The UI of Atom is pretty complex in some spots though (especially with the new Shadow DOM feature, see all the topics mentioning it here on Discuss) so it may take a bit of digging to figure out what you're looking at.
The GitHub team has mentioned that they have someone working on the documentation for Atom and they'll be releasing a new documentation repository when it gets closer to completion. Until then, feel free to mention things that aren't clear here or in bugs on https://github.com/atom/atom/issues/new.
I totally understand getting frustrated and I hope that we can help you get further along