[gedy]

That's a great point and something I've seen dealing with multiple look and feels at a single company. While not quite as far as what you suggest, we've had good success building components with no custom CSS at all and instead use Bootstrap classes exclusively. This allows a lot of freedom for the design system to look at it needs, but encapsulates the "hard parts" as you list. We've struggled with react-select as well, but were able to mostly specify various props to use Bootstrap classes. Not a sexy approach, but it works.