I dont think p5js is designed to build webpages or to be an IDE to do so. I see p5js editor (referring to the p5js PDE editor) to be used to design sketches. When it comes to working in a site, I see p5js as an element in the page consisting of a canvas. If I were to design an actual webpage, I would work with a single IDE like VS Code. I guess sublime, notepad++ or probly atom would work as well.
Not usre if any other online p5js editor has this capability to edit css.
Kf