text_style library Null safety

Based on the TextStyle class in the Flutter SDK.

Since we can't look up the ThemeData class in the build method there are helpful methods to get the design tokens for Material 3 text styles and optional overrides.

- Theme.of(context).textTheme.displayLarge.copyWith(fontSize: 24);
+ TextStyle.displayLarge(fontSize: 24);

Classes

BodyLarge
BodyMedium
BodySmall
DisplayLarge
DisplayMedium
DisplaySmall
HeadlineLarge
HeadlineMedium
HeadlineSmall
LabelLarge
LabelMedium
LabelSmall
TextStyle
TitleLarge
TitleMedium
TitleSmall