Oh, I guess I cannot edit it anymore.

*EDIT* for the above.

What are some typical defaults that I should start with for things like margin, padding, font size? What sort of items should I be initializing in ::root then overriding later?

I copied this from a specific project, so there might be a couple things missing, but here is generally what I use

https://gist.github.com/bdougherty/404b4ca33dfdbff48614b454f...

This might be helpful.

https://necolas.github.io/normalize.css/

Design decisions, though, are ultimately up to your taste and judgement.