google_fonts.dart

Text(
  'This is Google Fonts',
  style: GoogleFonts.lato(
    textStyle: Theme.of(context).textTheme.headline4,
    fontSize: 48,
    fontWeight: FontWeight.w700,
    fontStyle: FontStyle.italic,
  ),
),

// OR You can directly set a text theme

final textTheme = Theme.of(context).textTheme;

MaterialApp(
  theme: ThemeData(
    textTheme: GoogleFonts.latoTextTheme(textTheme).copyWith(
      body1: GoogleFonts.oswald(textStyle: textTheme.body1),
    ),
  ),
);