It's a set of properties/attributes commonly referred to as "style" (generic, not the CSS one). Or call it look & feel.

So you're saying the site screams "Claude's style!" to you. Not too different from "I know a WordPress site when I see one".

The sepia-tinted color scheme and the way LLMs overuse rounded corner tiles to the absolute death is part of it. Really what it comes down to, is that these LLMs have a design vocabulary of like 15 traits. None of the websites they generate have every single trait, but they are composed of some subset of 10 such traits, so it's still overwhelmingly obvious the moment you open the page, even though each one is slightly different. Humans might use 1~5 of those traits, but they'll also be mixed in with a much wider set of traits with more individuality to them.

Most AI-generated sites have coopted the Inter font, and usually they go for a dark colour scheme, the rounded tiles, a container representing a macOS-styled terminal with code, and, the thing that annoys me the most, divs that fade is as you scroll.