You seem to have misunderstood; Remix promotes the use of bog-standard CSS. Literally nothing about it in any way inhibits reuse of styles across the app.
Not sure where you're going in critiquing styled-components.
Well, does it keep the styles with the components, or does it expect devs to create a separate folder for shared styles? If there are lots of shared styles, that equates to having two sets of components: one React, the other CSS.