CSS is old tech based on old ideas. It still has its place but needs to be superseded.
Keep in mind that even Rebass uses CSS to style.
The idea that we're trying to build medium-scale application UIs using a tool set intended to mark up documents with modest formatting requirements is still as crazy as it always was. Pragmatically, we can do it and make stuff work, and because of the compelling advantages of browser ubiquity and the distributed nature of the Web, we put up with the shortcomings, at least for now.
However, none of HTML, CSS or JS is actually very good for full application development. We had better technologies for native applications decades ago. If mainstream operating systems had solved their application deployment, maintenance and security problems within the same time frame, web apps would probably never have existed, and developing modern front-end code for distributed software would probably be much, much easier.