Why isnt there dark theme for GitHub pages with only readme file?

This is the GitHub repo, and this is the gh-pages site. While it works just as expected, I don’t know how to make the site have a dark theme like its possible for normal GitHub

Dark/Light theme is a feature of GitHub’s WebUI template, it’s not related to GitHub Pages websites. Websites have their own templates (in case you use Jekyll and pick one of the default templates).

Nothing prevents you from adding to your website a template that supports light and dark versions, but you’ll have to implement that yourself, via a CSS template that allows toggling between the two.