Unable to completely disable theme or use custom default layout

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This issue affects

[ ] The site generated by GitHub Pages
[x] Building sites locally

The GitHub Pages Gem is intended to help users replicate the GitHub Pages build environment locally. If your issue affects both the hosted version and the version previewed locally, you may be better suited reporting seeking support in other forums.

What did you do (e.g., steps to reproduce)

github-pages forces me to use the jekyll-theme-primer even if I set no theme in my _config.yml file. There is no way to completely disable the theme.

Also, adding a default.html layout and specifying it or not in my source file’s frontmatter has no effect.

This issue is related to: https://github.com/jekyll/jekyll/issues/9143

What did you expect to happen?

I expect to have a way to completely disable the theme so I can just write my own HTML, like setting theme: false in _config.yml. But there is no way to do this.

I also expect my _layouts/default.html file to be used instead, but it is never used.

What happened instead?

Either jekyll-theme-primer always takes over no matter what, or I only get my source file rendered without any layout HTML. In the case of index.md, when served locally, I sometimes even get a Webrick directory listing.

Additional information

Link to the live site (if applicable): N/A
Link to the source repo (if applicable): Can make one if needed.

github / pages-gem