custom dark light theme switch system for react app build with material ui design

app.js

import ContentApp from './Content/ContentApp'
import ThemeHandler from './Theme/ThemeProvider';
import CssBaseline from '@material-ui/core/CssBaseline';

function App() {
  return (
    <ThemeHandler>
      <CssBaseline />
      <ContentApp />
    </ThemeHandler>
  );
}

export default App;