title: React with Material UI not finding custom font
body: <p>I have my mui theme defined like this:</p>
  
  <pre><code>export default createMuiTheme({
    typography: {
      fontFamily: '"Nunito Sans", "Helvetica", "Arial", sans-serif',
      fontWeightLight: 300,
      fontWeightMedium: 600,
      fontWeightRegular: 400
      }
    }
  });
  </code></pre>
  
  <p>I have the fonts being loaded in using <code>App.css</code> from local files. I can see from the network requests that these files are being found. </p>
  
  <pre><code>/* latin */
  @font-face {
    font-family: 'Nunito Sans';
    font-style: normal;
    font-weight: 300;
    src: local('Nunito Sans Light'), local('NunitoSans-Light'), 
    url(../assets/font/pe03MImSLYBIv1o4X1M8cc8WAc5tU1E.woff2) format('woff2');
    unicode-range: U+0000-00FF, U+0131, U+0152-0153, U+02BB-02BC, U+02C6, U+02DA, U+02DC, U+2000-206F, U+2074, U+20AC, U+2122, U+2191, U+2193, U+2212, U+2215, U+FEFF, U+FFFD;
  }
  
  /* latin */
  @font-face {
    font-family: 'Nunito Sans';
    font-style: normal;
    font-weight: 400;
    src: local('Nunito Sans Regular'), local('NunitoSans-Regular'), url(../assets/font/pe0qMImSLYBIv1o4X1M8cce9I9s.woff2) format('woff2');
    unicode-range: U+0000-00FF, U+0131, U+0152-0153, U+02BB-02BC, U+02C6, U+02DA, U+02DC, U+2000-206F, U+2074, U+20AC, U+2122, U+2191, U+2193, U+2212, U+2215, U+FEFF, U+FFFD;
  }
  
  /* latin */
  @font-face {
    font-family: 'Nunito Sans';
    font-style: normal;
    font-weight: 600;
    src: local('Nunito Sans SemiBold'), local('NunitoSans-SemiBold'), 
    url(../assets/font/pe03MImSLYBIv1o4X1M8cc9iB85tU1E.woff2) format('woff2');
    unicode-range: U+0000-00FF, U+0131, U+0152-0153, U+02BB-02BC, U+02C6, U+02DA, U+02DC, U+2000-206F, U+2074, U+20AC, U+2122, U+2191, U+2193, U+2212, U+2215, U+FEFF, U+FFFD;
  }
  </code></pre>
  
  <p>The issue is that the UI is falling back to "Helvetica". I can see no reason why "Nunito Sans" is not being used by MUI. Annoyingly this setup was working fine before and is now failing. Anyone any ideas why this might not work? Thanks!</p>