While searching for blog themes, I stumbled across Gregory Gundersen's blog, which caught my eye at first sight. The original author did not declare any open source license:
Feel free to copy my site. However, I think it’s more important to understand a basic design principle so that you can modify my template as you please.
I personally prefer serif fonts, so I simply modified the css file, along with the blog's personal information, and all the rest of the code remains the same.
Since the Jekyll Ruby library supported by GitHub Pages is not very complete, e.g Liquid tag, many of the Ruby libraries that this blog relies on cannot be built for GitHub Pages.
The relatively easier route would be to ditch GitHub Pages entirely for a third-party player like Netlify, Cloudcannon, etc depending on your use-case. I ended up deploying this blog on vercel.