Don't get me wrong, I had the same reaction as well when I first saw it, but the HTML/CSS/JS divide only really makes sense when you're making a traditional HTML document. When we're making component-ised UI elements, separating out that code across HTML templates, a JS file and a CSS file doesn't actually make that much sense.
I don't personally use JS-ed CSS like this, but I also don't think it's a totally insane idea.
Fully encapsulating components is one thing, especially if their encapsulated implementations use shared stylings under the hood. Anything else is repetitive copy-pasta that will be difficult to maintain.
Not at all. In fact I'd argue it's just as simple if not even simpler than having plain old CSS in that use case. You could easily have brand colours in either a theme or config file and there you go: one line of change. As others have said it's a lot easier than it sounds and styled-compenents makes a lot of sense especially in a world where a lot of places are preprocessing their stylesheets with SASS or LESS.
The problem you're describing shouldn't actually exist in a well-structured component system based on a well-considered design system.
Secondly, nothing is stopping you importing constants representing brand colours, spacing and typography from other modules, then using them in your component styles. In fact, this is what people actually do.
With styled-components you define the theme once, and then just use those variables in all of your components.
With plain CSS you end up sometimes repeating the same value as a color or a background-color or a fill or stroke and then you need it to generate a dynamic value in JS and suddenly you either have to come up with clever tricks involving semi-transparent overlays or you copy the same value from your CSS into your JS and now everything's horrible.
Styled-components and "CSS in JS" solve that. Sure, CSS variables solve that too (except when you need the variables in JS), but out here in the real world people have to support browsers that don't support CSS variables.