But idk what React codebases you've been in, the bog-standard, overwhelmingly popular bootstraps and guides since way before Svelte ever existed had some pretty nice style solutions and at the minimum importing CSS.
React has some of the most gorgeous style solutions around (I should know having built the best one) and has since forever ago. Svelte is less popular than many React starter kits with I'd argue much nicer DX.
CSS needed drastic changing too, and I think if you looked on average total CSS knowledge is higher than ever, you're just seeing frontend become popular and wide, different types of sub-specialties at this point.