Video Player - how to center the main div?

Hi all,
I’m trying to run https://www.codecademy.com/courses/react-102/projects/video-player in VS Code. It runs well, but I can’t center the global div. All elements (title, menu & video) are stucked to the left.
Even after adding
.center-div { margin: 0 auto; width: 800; } in index.css,
import './index.css'; in App.js,
and
render() { return ( <div className="center-div"> <h1>Video Player</h1> <Menu chooseVideo={this.chooseVideo} /> <Video src={this.state.src} /> </div> ); } in App.js

How can I get all elements centered in the page ?