The hugo new theme mytheme command generates these partial templates:

_partials/
├── head/
│   ├── css.html
│   └── js.html
├── footer.html
├── head.html
├── header.html
├── menu.html
└── terms.html

The js.html partial demonstrates the js.Build function.

Let's make the css.html partial symmetrical, demonstrating the css.Build function. We would keep this very simple, moving the existing header and footer rules into their own component files, them @import them into main.css.