// DRY - color defined once, reused everywhere
final primaryColor = Color(0xFF1565C0);
Container(color: primaryColor, child: Text('Header')),
Container(color: primaryColor, child: Text('Footer')),

Container(
  color: Theme.of(context).primaryColor,
  child: Text('Hello', style: TextStyle(color: Theme.of(context).colorScheme.onPrimary)),
)

MaterialApp(
  theme: ThemeData.light(),
  darkTheme: ThemeData.dark(),
  themeMode: ThemeMode.dark,
)

class PersonModel {
  final String name;
  final int age;
  final DateTime birthDate;
  PersonModel({required this.name, required this.age, required this.birthDate});
}

int calculateDaysAlive() {
  final today = DateTime.now();
  return today.difference(person.birthDate).inDays;
}

dependencies:
  intl: ^0.19.0

class DaysCalculator {
  final DateTime birthDate;
  DaysCalculator(this.birthDate);
  int calculateDaysAlive() => DateTime.now().difference(birthDate).inDays;
  bool isOver1000DaysOld() => calculateDaysAlive() > 1000;
}