You’re kinda late to the party. 15 years ago that was the way to build UIs, but componentization changed that. Now we reason about UIs as blocks, not as pages, so collocation of logic, markup, and style makes the most sense.

Not to say that every component should be unique, generic components can be built in an extensible way, and users can extend those components while applying unique styling.

Theming is also a solved issue through contexts.

Reducing coupling was never a good idea. Markup and styling are intrinsically linked, making any change to the markup most likely will require changes to the styling, and vice versa. Instead of pretending we can separate the two, modern UI tools embrace the coupling and make building as efficient as possible.