Probably just a css file that styles the html elements only without using any classes

No style attributes. You just use HTML markup and use a classless CSS framework to take care of making it look nice. My favorite is Marx, but there are others you can find here: https://github.com/dbohdan/classless-css

Water.css, MVP.css, sakura, and Tacit are among the most popular.

I've always liked Pico myself.