setTheme = () => {
    let element = document.getElementById('doc');
    // element.setAttribute('class','dark');
    element.classList.toggle('dark');
}

  render() {
    return (
      <div id='doc' class='light'>
        <h1>Hey</h1>
        <button id='btn' onClick={this.setTheme}>Click</button>
      </div>
    )
  }