If you did step 3 - you will see, that there is not much to do with CSS and Javascript. If you want to keep your GitHub Page, go ahead and add some styles as stated in this tutorial.
You can also upload a HTML based website, independently from the GitHub Markdown.
- Clone the repository, so you have your files localy
- Download a free template, like this one
- extract the files, and open the directory with Atom
- open the website localy by right clicking the file
about.htmland chooseopen in Browser - see, the template is fancy responsive
- review the code
- edit the files
- rename about.html to index.html globally in code, too
- do not remove the credits at the footer
- you might want to remove the contact form, since we do not have a server for actually sending emails. But if you are curious, use this service for static contact forms.
- replace the images with correct sizes ones
- free images at wikimedia Commons (mind the license)
- Example Profile Image
- push the code to the GitHub Page repository