There are some videos on youtube.
I know it's not gonna be the answer you are looking for, but maybe some of those videos can point you in the right direction.
But from what I understand, there is no pinpoint diagram of how styling works, etc.
Right-Clicking on something and choosing browser inspect can help you with styling as well.
it's always best to use the built-in customization if you can... ends up with less lines of code that simply won't function if you over-write it in extra.css.
agreed, it's sometimes hard to find things and using the template sass variables can get confusing to pinpoint.
Maybe this will help a bit:
https://xenforo.com/docs/xf2/styles/
I was gonna post the help doc also but had to run out of the house to do something lol. Great, advise!
We have a Ton of Developers who create Styles for xenforo.
Would be amazing if one of them was sooo kind, as to give a basic tutorial thread on styling for new members.