How to change the entire theme’s text color in Flutter?

I think TextTheme.apply is what you want. bodyColor will be applied to headline, title, subhead, button, body1, and body2. displayColor will be applied to display1 through display4, and caption. If you specify both bodyColor and displayColor and use the same color value, that will effectively change text colors on all text styles.

Example:

final newTextTheme = Theme.of(context).textTheme.apply(
  bodyColor: Colors.pink,
  displayColor: Colors.pink,
);