You can create a GitHub Pages site with the Jekyll Theme Chooser using a new or existing site repository.
You can change your theme using the theme selector at anytime.
After creating your site with the Jekyll Theme Chooser, you can customize your site. For more information, see "Further customizing your Jekyll site."
Using the Jekyll Theme Chooser with a new repository
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Create a new repository with a README. Your new repository can be public or private. If your repository is private, your published site is still public.
To create a User or Organization Pages site, you must name your repository
username.github.io, using the name of the user or organization that owns the repository. If you choose a different repository name, then the Jekyll Theme Chooser automatically creates a Project Pages site for you. For more information, see "User, Organization and Project Pages." Under your repository name, click Settings.

Under "GitHub Pages", click Choose a theme.

On the theme selection page, review your theme options. To choose a theme, click the theme you want and then click Select theme.

After selecting a Jekyll theme, you'll be prompted to edit your site's README.md file. Click Cancel to edit it later. If you'd like to edit it now, see "Editing files in your repository" and commit your changes to the
masterbranch.
Under your repository name, click Settings.

To preview your site, under "GitHub Pages" click the URL to your GitHub Pages site. For troubleshooting help, see "Troubleshooting GitHub Pages builds."

Using the Jekyll Theme Chooser with an existing repository
If you have an existing Jekyll site repository you want to use on GitHub with the Jekyll Theme Chooser, you may have to reformat your Markdown files or layout for your site to render properly. If your site repository has a .nojekyll file, you must delete it before you can use the Jekyll Theme Chooser.
Navigate to your GitHub Pages site's repository.
Under your repository name, click Settings.

Under "GitHub Pages", click Choose a theme.

On the theme selection page, review your theme options. To choose a theme, click the theme you want and then click Select theme.

Under your repository name, click Settings.

To preview your site, under "GitHub Pages" click the URL to your GitHub Pages site. For troubleshooting help, see "Troubleshooting GitHub Pages builds."

Further customizing your Jekyll site
If you'd like to change the CSS or HTML layout of your Jekyll theme, see "Customizing CSS and HTML in your Jekyll theme."
Add more pages to your site
To learn more about adding other pages or posts to your GitHub Pages site, see Jekyll's official documentation on creating pages or posts. Other pages or posts you add to your site will share your Jekyll theme's style and layout.
Customize your site's URL
To learn more about customizing your site's URL, see "Using a custom domain with GitHub Pages."
Choose a different publishing source or branch
If you would like to publish your GitHub Pages site using the gh-pages branch or from a docs folder in your repository, see "Configuring a publishing source for GitHub Pages."