How can I customize react-app-rewired config in CRA v5 to add a postcss plugin, for example this one? I checked react-app-rewired documentation and github issues, but didn't find out how to do that.
You can try the react-app-rewire-postcss library that does exactly what you need. There might be issues with PostCSS version, but you can definitely use the library's code as a hint about how what you need can be implemented.