pages/_document.tsx

import Document, { Html, Head, Main, NextScript } from 'next/document';
import { ServerStyleSheet } from 'styled-components';

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();
    }
  }

  render() {
    return (
      <Html>
        <Head>
          <link
            rel='stylesheet'
            href='https://fonts.googleapis.com/css2?family=Assistant:wght@400;700&family=Open+Sans:wght@700&display=swap'
          />
        </Head>
        <body>
          <Main />
          <NextScript />
        </body>
      </Html>
    );
  }
}

export default MyDocument;