You can install a Jekyll theme to customize the design of your GitHub Pages site without having to edit or copy CSS files. Jekyll themes package the files you need to change your site design, including any files that are stored in your assets folder, like SCSS, SASS, images, and CoffeeScript.
You can add a Jekyll theme to your GitHub Pages site by manually editing your site's _config.yml file or by using the Jekyll Theme Chooser.
When you manually edit your site's _config.yml file to add a Jekyll theme to your GitHub Pages site, you can choose any of the officially supported themes, or any open source Jekyll theme hosted on GitHub. For more information, see "Adding a Jekyll theme to your GitHub Pages site."
When you use the Jekyll Theme Chooser to add a theme to your GitHub Pages site, or to change your theme, you can choose one of the supported themes. For more information, see "Adding a Jekyll theme to your GitHub Pages site with the Jekyll Theme Chooser."
By default, the Jekyll Theme Chooser uses the
master branch to publish GitHub Pages sites. It also uses a default URL based on the name of the repository where your site exists, such as
username.github.io/repository-project-name. For more information about naming GitHub Pages repositories, see "User, Organization, and Project Pages."