developer-roadmap/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()
    }
  }
}