next.js/examples/with-global-stylesheet/pages/index.js

import React from 'react'

import stylesheet from 'styles/index.scss'
// or, if you work with plain css
// import stylesheet from 'styles/index.css'

export default () =>
  <div>
    <style dangerouslySetInnerHTML={{ __html: stylesheet }} />
    <p>ciao</p>
  </div>