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

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

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