I would make it clear in the landing page that the components are for demonstration purposes by adding a title like "For example" before them.
The above the fold looks a bit packed right now. I would leave the login box out until user presses top right as it's for retentive users only.
I don't think the demo should overpower the landing page.
And then it goes straight into themes. If I'm a Rails developer I'm not looking at theming, I'm looking for a conventional UI system that fits into Rails - stimulus, Hotwire, all that.
As far as I know, this site so far is just a bunch of specialised scaffolds for certain use-cases, but Rails itself has been capable of that the entire time.
You can either take the pages and tweak them for your own use case, or just use the UI components and skip the theme entirely. If you get a chance, try the free Ruby gem to see what I mean.
The login box is maybe confusing or maybe I'm misunderstanding you, it's actually UI for a login box, not actually where you login. I agree this area could be tightened up.