If you try to use a custom theme for which Gem is not whitelisted by GitHub, you may need to declare the custom theme in your _config.yml file as remote_theme: github_user/repo and remove it from the Gemfile .
Here is my Gemfile :
source "https://rubygems.org" gem 'github-pages', group: :jekyll_plugins
And the interesting part of my _config.yml :
remote_theme: pawamoy/jekyll-readthedocs
To create locally, just run bundle exec jekyll serve .
Learn more about the GitHub help page for Jekyll themes .
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