Forked repo doesn't pick up remote theme for GitHub pages

I have forked a repository that is configured to become a GitHub Pages site with a custom theme. I noticed that I cannot preview the exact outcome with just preview plugins for my local editor, and I prefer not to set up a local configuration so I decided to try and create my own GitHub Pages site so I can tinker with the preview until I’m satisfied before I do a pull request to the owner of the repository I forked.

I deleted the CNAME file so GitHub won’t try to update the owner’s site (yes I know that means I can’t create a pull request anymore, but I’ll fix that), but now my version of the site doesn’t pick up the custom theme.

I hope someone can tell me what’s wrong.

Here is my fork: