People with write permissions for a repository can add a theme to a site using Jekyll.
Adding a theme 
- Navigate to _config.yml. 
- Add a new line to the file for the theme name. - To use a supported theme, type theme: THEME-NAME, replacing THEME-NAME with the name of the theme as shown in the_config.ymlof the theme's repository (most themes follow ajekyll-theme-NAMEnaming convention). For a list of supported themes, see "Supported themes" on the site. For example, to select the Minimal theme, typetheme: jekyll-theme-minimal.
- To use any other Jekyll theme hosted on , type remote_theme: THEME-NAME, replacing THEME-NAME with the name of the theme as shown in the README of the theme's repository.
 
- To use a supported theme, type 
Customizing your theme's CSS 
- Create a new file called /assets/css/style.scss. 
- Add the following content to the top of the file: 
- Add any custom CSS or Sass (including imports) you'd like immediately after the - @importline.
Customizing your theme's HTML layout 
- On , navigate to your theme's source repository. For example, the source repository for Minimal is https://github.com/pages-themes/minimal. 
- In the _layouts folder, navigate to your theme's default.html file. 
- Copy the contents of the file. 
- Create a file called _layouts/default.html. 
- Paste the default layout content you copied earlier. 
- Customize the layout as you'd like.