override theme's `default.html` or equivalent

### This issue affects

- [x] 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](https://github.com/github/pages-gem/blob/master/docs/SUPPORT.md).*

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

- using a fork of [foam-template](https://github.com/foambubble/foam-template)
- gemfile has only two deps, this one and webrick

### What did you expect to happen?

- override theme defaults
- customize theme's default.html
- add `meta` tags to html

### What happened instead?

- no override possible / if i create a `default.html` in `_layouts` it is ignored

### Additional information

* Link to the live site (if applicable): https://evenzero.in/singularity/
* Link to the source repo (if applicable): https://github.com/surajsharma/singularity