how to use create mui theme in div code example

Example: use theme in component mui

import React from "react";
import { makeStyles } from "@material-ui/core/styles";

const useStyles = makeStyles((theme) => ({
  root: {
    backgroundColor: theme.palette.secondary.main
  }
}));

export default function App() {
  const classes = useStyles();
  return (
    <div className={classes.root}>
      <h1>Hello CodeSandbox</h1>
      <h2>Start editing to see some magic happen!</h2>
    </div>
  );
}