Let me know if you need anything else to help me out here. I would love to move my site to Jekyll instead of WordPress if possible and I have all of my posts and assets all set locally but I don’t want to do this if it is not going to show the themes properly.
You can pretty quickly isolate out Github Pages as the root cause by trying a different host that supports the full range of plugins and Jekyll versions. Netlify is a popular host amongst the Jekyll crowd - last time I set up a Jekyll site using Netlify it only took me a few minutes.
Just looked at your site up on GitHub Pages and it looks fine to me with CSS loading properly. Perhaps you’ve fixed the issue?
I did notice you have a bunch of posts with .html instead of .md
That would explain why they aren’t showing up. I don’t think you can use .html files with front matter. If you change the extensions to .md you can leave the html in the body as Kramdown will parse it as is.