function Dialog() {
  return React.createPortal( <div>Dialog contents</div>, document.body )
}

class Modal extends React.Component {
 render() {
  const {
   visible,
   onClose
  } = this.props
  return visible && ReactDOM.createPortal(<StyledModalRoot>
   <div className="box">
    Content
    <br/>
    <button onClick={onClose}>Close</button>
   </div>
  </StyledModalRoot>, document.body)
 }
}

class App extends React.Component {
 state = {
  visibleModal: false
 }
 showModal = () => this.setState( { visibleModal: true } )
 handleCloseModal = () => this.setState( { visibleModal: false } )
 render() {
  const { visibleModal } = this.state
  return <div style={{padding: '20px'}}>
  <button onClick={ this.showModal }>Show Modal</button>
  <Modal visible={visibleModal} onClose={ this.handleCloseModal } />
 </div>
 }
}

const StyledModalRoot = styled.div`
 position: fixed;
 z-index: 1001;
 left: 0;
 top: 0;
 display: grid;
 place-items: center;
 width: 100%;
 height: 100%;
 background: rgba( 0, 0, 0, 0.2 );

 >.box {
  position: relative;
  display: grid;
  place-items: center;
  width: 80%;
  height: 80%;
  background: white;
  border-radius: 10px;
  box-shadow: 0px 3px 5px -1px rgba(0,0,0,0.2), 0px 5px 8px 0px rgba(0,0,0,0.14), 0px 1px 14px 0px rgba(0,0,0,0.12);
 }
`