title: How do I use google fonts on defining a theme in flutter
body: <p>I am new to flutter.</p>
  <p>I installed <a href="https://pub.dev/packages/google_fonts" rel="noreferrer">this</a> library to be able to use google fonts.</p>
  <p>Now I need to do it in the theme data definition, and tried like this but it's not allowed</p>
  <pre><code>ThemeData appTheme() {
    return ThemeData(
      ...
      fontFamily: GoogleFonts.openSans(),
    );
  }
  </code></pre>
  <p>How do I do this? Thank you very much</p>