class.js

const styleSheet = createStyleSheet('MyComponent', () => {
  return {
    base: {
      margin: 32
    },
    button: {
      '@scope': 'base',
      minWidth: 64
    }
  }
});

composition-support.js

const styleSheet = createStyleSheet('MyComponent', () => {
  return {
    base: {
      margin: 32,
      '& Button': {
        base: { minWidth: 64 }
      }
    }
  }
});