CSSModules.js

// Component JS file
import React from 'react'
import s from './style.scss' // <--- Imported into JS file

const SomeComponent = () => (
  <div className={s.container}> // <---- Can use more simple classes as they're automatically name-spaced
    <span className={s.text}>Hello!</span>
  </div>
)

// CSS file
.container {
  display: flex;
  align-items: center;
  justify-content: center;
}

.text {
  color: green;
}

/*------------------------------------------------------------*/

// Component JS file
import React from 'react'
                          // <--- No import, included via main.css

const SomeComponent = () => (
  <div className='some-container'> // <--- Need to name-space classes to avoid collisions
    <span className='some-text'>Hello!</span>
  </div>
)

// styles.scss
.some-container {
  display: flex;
  align-items: center;
  justify-content: center;
}

.some-text {
  color: green;
}