add jss golbal with material-ui

jss-global-mui.js

import MonsteredRegular from '~/public/fonts/Montserrat/Montserrat-Regular.otf'

const GlobalCss = withStyles({
  // @global is handled by jss-plugin-global.
  '@global': {
    // You should target [class*="MuiButton-root"] instead if you nest themes.
    '.MuiButton-root': {
      fontSize: '1rem',
    },
    '.exampleClass': {
      backgroundColor: 'blue',
    },
    '@font-face': {
      fontFamily: 'Montserrat',
      fontStyle: 'normal',
      fontWeight: 400,
      fontDisplay: 'swap',
      src: `url(${MonsteredRegular})`,
    },
  },
})(() => null)


render() {
    return (
      <Suspense fallback={<Loader />}>
          <GlobalCss />
          <div className={classes.rootContainer}>
            <CssBaseline />
           </div>
      </Suspense>
    )
  }
}