Шрифт по умолчанию для MaterialApp описан в
/flutter/packages/flutter/lib/src/material/typography.dart
на iOS, по умолчанию TextTheme -
static const TextTheme whiteCupertino = TextTheme(
display4 : TextStyle(debugLabel: 'whiteCupertino display4', fontFamily: '.SF UI Display', inherit: true, color: Colors.white70, decoration: TextDecoration.none),
display3 : TextStyle(debugLabel: 'whiteCupertino display3', fontFamily: '.SF UI Display', inherit: true, color: Colors.white70, decoration: TextDecoration.none),
display2 : TextStyle(debugLabel: 'whiteCupertino display2', fontFamily: '.SF UI Display', inherit: true, color: Colors.white70, decoration: TextDecoration.none),
display1 : TextStyle(debugLabel: 'whiteCupertino display1', fontFamily: '.SF UI Display', inherit: true, color: Colors.white70, decoration: TextDecoration.none),
headline : TextStyle(debugLabel: 'whiteCupertino headline', fontFamily: '.SF UI Display', inherit: true, color: Colors.white, decoration: TextDecoration.none),
title : TextStyle(debugLabel: 'whiteCupertino title', fontFamily: '.SF UI Display', inherit: true, color: Colors.white, decoration: TextDecoration.none),
subhead : TextStyle(debugLabel: 'whiteCupertino subhead', fontFamily: '.SF UI Text', inherit: true, color: Colors.white, decoration: TextDecoration.none),
body2 : TextStyle(debugLabel: 'whiteCupertino body2', fontFamily: '.SF UI Text', inherit: true, color: Colors.white, decoration: TextDecoration.none),
body1 : TextStyle(debugLabel: 'whiteCupertino body1', fontFamily: '.SF UI Text', inherit: true, color: Colors.white, decoration: TextDecoration.none),
caption : TextStyle(debugLabel: 'whiteCupertino caption', fontFamily: '.SF UI Text', inherit: true, color: Colors.white70, decoration: TextDecoration.none),
button : TextStyle(debugLabel: 'whiteCupertino button', fontFamily: '.SF UI Text', inherit: true, color: Colors.white, decoration: TextDecoration.none),
subtitle : TextStyle(debugLabel: 'whiteCupertino subtitle', fontFamily: '.SF UI Text', inherit: true, color: Colors.white, decoration: TextDecoration.none),
overline : TextStyle(debugLabel: 'whiteCupertino overline', fontFamily: '.SF UI Text', inherit: true, color: Colors.white, decoration: TextDecoration.none),
);
на Android, по умолчанию TextTheme -
static const TextTheme whiteMountainView = TextTheme(
display4 : TextStyle(debugLabel: 'whiteMountainView display4', fontFamily: 'Roboto', inherit: true, color: Colors.white70, decoration: TextDecoration.none),
display3 : TextStyle(debugLabel: 'whiteMountainView display3', fontFamily: 'Roboto', inherit: true, color: Colors.white70, decoration: TextDecoration.none),
display2 : TextStyle(debugLabel: 'whiteMountainView display2', fontFamily: 'Roboto', inherit: true, color: Colors.white70, decoration: TextDecoration.none),
display1 : TextStyle(debugLabel: 'whiteMountainView display1', fontFamily: 'Roboto', inherit: true, color: Colors.white70, decoration: TextDecoration.none),
headline : TextStyle(debugLabel: 'whiteMountainView headline', fontFamily: 'Roboto', inherit: true, color: Colors.white, decoration: TextDecoration.none),
title : TextStyle(debugLabel: 'whiteMountainView title', fontFamily: 'Roboto', inherit: true, color: Colors.white, decoration: TextDecoration.none),
subhead : TextStyle(debugLabel: 'whiteMountainView subhead', fontFamily: 'Roboto', inherit: true, color: Colors.white, decoration: TextDecoration.none),
body2 : TextStyle(debugLabel: 'whiteMountainView body2', fontFamily: 'Roboto', inherit: true, color: Colors.white, decoration: TextDecoration.none),
body1 : TextStyle(debugLabel: 'whiteMountainView body1', fontFamily: 'Roboto', inherit: true, color: Colors.white, decoration: TextDecoration.none),
caption : TextStyle(debugLabel: 'whiteMountainView caption', fontFamily: 'Roboto', inherit: true, color: Colors.white70, decoration: TextDecoration.none),
button : TextStyle(debugLabel: 'whiteMountainView button', fontFamily: 'Roboto', inherit: true, color: Colors.white, decoration: TextDecoration.none),
subtitle : TextStyle(debugLabel: 'whiteMountainView subtitle', fontFamily: 'Roboto', inherit: true, color: Colors.white, decoration: TextDecoration.none),
overline : TextStyle(debugLabel: 'whiteMountainView overline', fontFamily: 'Roboto', inherit: true, color: Colors.white, decoration: TextDecoration.none),
);
, и вы можете получить семейство шрифтов с помощью следующего кода
DefaultTextStyle.of(context).style.fontFamily