Add google_fonts to your pubspec.yaml file and add it to your Text widget like this:
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Text('Example Text', style: GoogleFonts.inter(
textStyle: TextStyle(
fontSize: 40, color: primaryColor,
fontWeight: FontWeight.w700,))
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MaterialApp(
theme: ThemeData(
textTheme: GoogleFonts.montserratTextTheme(
Theme.of(context).textTheme,
),
),
)