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.
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 Enterprise. For more information, see "Adding a Jekyll theme to your GitHub Pages site."