personal-website/pages/_document.tsx

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

class Doc extends Document {
	static async getInitialProps(ctx: DocumentContext) {
		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();
		}
	}
}

export default Doc;