Custom.css - where can I find the DOM?

HI all,
I am trying to adjust <custom.css> to my needs, and there are lots of examples our there. But as of now, I spent like 2 hours digging through examples in order to find out how to do more complex things. E.g. I found here how I can highlight an entire row.

But I could have done the same thing in 2 minutes if I knew the DOM. From the above link I learned that there appear to be attributes named “data-ref”, and some element class “flex-row”, but I have no idea where the devs of such themes gained that knowledge from (and I have a hard time believing they all reverse-engineered the existing CSS examples). There must be a way to either see the DOM of a Logseq document, or some example DOM, or just about anything, and I just failed to google that information so far…?

Thanks a lot!

I think people just open Developer Tools from Logseq app itself and check CSS classes and rules.