I solved the problem for my own component by removing all the code regarding the resolution of classnames in the origin RadioGroup.tsx file.
The strange thing is: the origin implementation of RadioGroup does so much about resolving classnames, and applying class names to each element in the component. But at the end it is not possible to adjust some basic styles by injecting additional class names. See this discussion. I wonder for what all the findStyleAsClassName calls are good for.
To me, it looks as if a lot of effort has been made to achieve a high degree of flexibility only internally, but without giving this flexibility to the outside.
But don’t get me wrong, the project is still brilliant.