Note: the official Hack Club theme packaged as plain CSS is hackclub/css. This one still works. It's just a little different.
hackclub.css is a classless CSS utility which applies the Hack Club theme styles to a given HTML page. 🔆 / 🌖 Color mode (light or dark) is automatically determined based on the system mode.
Insert the following into the <head> of your HTML file.
<link rel="stylesheet" href="https://cdn.jsdelivr.net/gh/exu3/hackclub.css@main/src/hackclub.min.css">
If you're looking for the Hack Club theme as an NPM package, you can find that here. (NPM) (Source)