## Summary

When `github-pages` is listed under the `group :jekyll_plugins` in the Gemfile, then **on regeneration**, a site has *its **`assets/css/style.css`*** overridden by one bundled with one of the themes in the gem.

This issue came to my notice while investigating `jekyll/jekyll#7854`.

## This issue affects

- [ ] The site generated by GitHub Pages ***`maybe.. maybe not..`***
- [x] Building sites locally

## Steps to reproduce
- Set up a Jekyll site with at least the following:
- Gemfile
```ruby
source 'https://rubygems.org'
gem 'github-pages', group: :jekyll_plugins
```
- **`assets/css/style.css`** with the following:
```css
h1 { color: red }
```
- An `index.html` that uses the above stylesheet. (either via a layout or directly)
```html




Test Site

Hello World

```
- Serve the site locally and preview at `http://localhost:4000/`
```
bundle exec jekyll serve
```
- Trigger a regeneration by **just adding content to the landing page**.
```diff

Hello World

+

Lorem ipsum dolor sit amet

```
---

## What did you expect to happen?

Expected the landing page to be rendered with the new content.

## What happened instead?
The following would illustrate better:

### Before regeneration:


### After regeneration: