Create a file named theme.dart in the root of the project
Example Code
theme.dart
import 'package:flutter/material.dart';
var appTheme = ThemeData(
bottomAppBarTheme: const BottomAppBarTheme(
color: Colors.black87,
),
brightness: Brightness.dark,
textTheme: const TextTheme(
bodyText1: TextStyle(fontSize: 18),
bodyText2: TextStyle(fontSize: 16),
button: TextStyle(
letterSpacing: 1.5,
fontWeight: FontWeight.bold,
),
headline1: TextStyle(
fontWeight: FontWeight.bold,
),
subtitle1: TextStyle(
color: Colors.grey,
),
),
buttonTheme: const ButtonThemeData(),
);
Apply it to the MaterialApp in the main.dart file
main.dart
import 'package:quizapp/theme.dart';
// ...
return MaterialApp(
theme: appTheme,
);