next.js/examples/with-styled-components/pages/_document.js

import Document from 'next/document'
import { ServerStyleSheet } from 'styled-components'

export default class MyDocument extends Document {
  static async getInitialProps (ctx) {
    const sheet = new ServerStyleSheet()

    const originalRenderPage = ctx.renderPage
    ctx.renderPage = () =>
      originalRenderPage({
        enhanceApp: App => props => sheet.collectStyles(<App {...props} />)
      })

    const initialProps = await Document.getInitialProps(ctx)
    return {
      ...initialProps,
      styles: [...initialProps.styles, ...sheet.getStyleElement()]
    }
  }
}