The lines in question:

    .stack > * + * {
      margin-block-start: 1.5rem;
    }

I got it immediately because I think I've been doing something similar for a while. I identified the problem it solves. I understand that if you haven't encountered the situation, it's probably hard to decipher.

It reads like: apply a top margin to all direct children that are not the first one.

I write it like this:

    .stack > :not(:first-child) {
      margin-top: 1.5rem;
    }

I probably should start using -block-start. I'll keep my selector though I think.

I didn't know about the default value parameter of --var, I'll probably use it too.