Introduction
(This post is for beginners of jekyll plug-ins and gem who want to learn about jekyll theme and jekyll-remote-theme)
A theme form help isolate the update of theme files and post files. For we can choose to update style files and the posts independently. But why Riinosite, and even most GitHubPage Blogs do not use theme?
How Jekyll Theme works
Theme is a type of Gem, which is managed by Bundler, a Ruby library management tool. Typically a gem is a pre-built package of files, which can be ruby codes or Jekyll theme template files. It can be initiate by jekyll new-theme jekyll-theme-awesome, you can check the jekyll theme doc. Normally when you declare the name of gems in Gemfile and use bundle install , bundler will scan your Gemfile config and download all gems, including your theme gem file.Then you have to declare theme: theme-name in _config.yml in your Jekyll to let your Jekyll site know that it can use a gem file downloaded by bundler whose name is 'theme-name' as the theme
But GitHub Page does not follow the rule above
First, GitHub Page has a gem whitelist, which means only the gems in this whitelist can be installed, where few theme gems is available, and the default one is the jekyll-theme-minimal, you can use theme: minimal in your _config.yml to enable this theme, which is also the default theme when generating a new jekyll blog. Therefore, although you can write, build and publish your own gem called jekyll-theme-mytheme, you can not use this gem in your GitHubPage.
However, there's a gem in this whitelist called jekyll-remote-theme, which allows you to use a new key in _config.yml called remote_theme. Where you can use a theme repo in same github hoster.
For example, if you have a theme in your-github-name/your-theme, and your blog is in your-github-name/your-github-name.github.io, you can use remote_theme: your-github-name/your-theme in your _config.yml.
Conclusion
To install a theme out of GitHubPage whitelist in GitHubPage, you have to install gem jekyll-remote-theme and fork or create a theme repo, then use remote_theme
While you can install any theme in your own jekyll site using theme in config file, if the theme gem is published in your bundler source.