main.js

import * as React from "react";
import * as ReactDOMServer from "react-dom/server";

function SomeComponent() {
  return React.createElement(StyledDiv, null, "Hello!");
}

const StyledDiv = styled.div`
  color: red;
`;

const rendered = ReactDOMServer.renderToString(React.createElement(StyledDiv));

webpack.config.js

const path = require("path");

module.exports = {
  mode: "development",
  target: "node",
  entry: "./main.js",
  output: {
    path: path.resolve(__dirname, "./"),
    filename: "[name].bundled.js"
  },
  resolve: {
    alias: {
      "react-dom/server": path.resolve(
        __dirname,
        "./node_modules/react-dom/server.browser.js"
      )
    }
  }
};