I had a website built, a MySpace competitor that had 3 themes inspired by street culture (graffiti, city streets and weed) and you could change the color of any of the themes, and the entire site would update. Your selected color and theme would be the basis for your profile so anyone visiting your profile would get your theme. There was an option to upload custom images too for the banner.
It supported secure custom HTML/CSS before MySpace too - no script tags allowed, certain tags had restricted attributes, many tags not allowed, all written in PHP.
Back then (2000s) it wasn’t some special skill, it was just what we knew how to do.
People get made fun of for calling out how modern frameworks made idiots capable of complex apps but we hav living proof of this, namely the “react is the last framework” crowd (they’re wrong, and stupid)
how did the people in 2001 know how to do crazy 3D web design, and how did we lose this so quickly