Flutter 2: How to extend ThemeData
Update in 2022: With the release of Flutter 3.0, we have a better solution:
Flutter 3: How to extend ThemeData
Extend ThemeData by ThemeExtension class introduced in Flutter 3.0.
The way to extend
ThemeData in Flutter has been a frequently asked question on StackOverflow: Is it possible “extend” ThemeData in Flutter.