Cool, hadn't followed a lot of css3+ support, including custom properties (variables)... I don't care for the separation of the SCSS source files and the components themselves though, to me this is a bit of an anti-pattern. Also, will mean that the entire CSS is loaded instead of just those components used. Use of rollup and scss directly is probably better off in the end though.
Seems a bit cleaner than the React bootstrap wrappers, and at a glance about as complete. MIT license.
Pretty decent overall, will want to keep this in mind for future projects. I still *really* like Material UI, but this is definitely a lighter/simpler library.