How to reuse the same style rule with multiple selectors

How about extend plugin (enabled by default)?

https://cssinjs.org/jss-plugin-extend

const styles = {
  buttonColor: {
    background: 'red'
  },
  button: {
    extend: 'buttonColor',
    fontSize: '20px'
  }
}

A simpler alternative that I feel is easier to parse would be to set the key to be a String by wrapping in quotes:

'.a, .b': {
   width: 100px; 
}