React

HTML

<div id="app"></div>

React

class TodoApp extends React.Component {
  static themes = {
    dark: 'dark-theme.css', 
    light: 'light-theme.css',
  };
  
  render() {
    return ReactDOM.createPortal(
	    (<link src={TodoApp.themes.dark}></link>),
      document.head
    );  
  }
}

ReactDOM.render(<TodoApp />, document.querySelector("#app"))