It is a legacy theme. It is not listed because it’s not compatible with Obsidian v1.0.0 and above. I would wait for them to update it, if I were you. There is an Issue on Github where someone commented that people are working on it.
But in general, if you want to be able to select a theme from the dropdown menu in Obsidian I think you need to add a manifest.json file, if there isn’t one already.
That file would have data like this:
"name": "Primary Theme",
"version": "some number here",
"minAppVersion": "minimum App version here",
"author": "Cecilia May"
Almost forgot, welcome to the Community! You will have lots of fun with Obsidian.
I added a manifest.json file, and renamed the theme.css one (it had a slightly different name initially) and it worked! I suspect the theme might break down in places because it’s not fully compatible with this version of Obsidian, but it’s still nice to see most of it in action. Thanks again!
And really enjoying being part of the Community already. : )