It sounds like you need to do Step 3 at the top under Basic Installation. You've already imported the main Core Parent style, correct? If so, proceed to step 3. DO NOT import Core Parent as a child of Core Parent. You need to add a style (as outlined in step 3)
Then for the dark style, I cover that in the "Using both a light and a dark version".
Yes, I imported the Core parent style and it works fine. I then imported the dark version as well, ie both are at the top level. I've imported the dark child under the dark parent, so the installation is correct sans the light version I'm asking about.
I did read those instructions all the way through already, but it wasn't clear to me, hence I asked my question.
I'll have a look at step 3 later, see if it makes sense to me now and get back to you.