Why theming creates consistent UI in Flutter - Build It to Prove It

Build: Themed Buttons Screen

This screen shows how using a theme creates a consistent look for buttons across the app.

Target UI

-------------------------
| Themed Buttons Screen  |
|-----------------------|
| [Primary Button]      |
| [Secondary Button]    |
| [Disabled Button]     |
-------------------------

Use ThemeData to define primary and secondary colors

Apply the theme to the MaterialApp

Create three buttons: primary, secondary, and disabled

All buttons should use the theme colors automatically

Disabled button should appear faded using theme's disabled color

Starter Code

Flutter

import 'package:flutter/material.dart';

void main() {
  runApp(const MyApp());
}

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Theming Demo',
      // TODO: Add theme here
      home: const ThemedButtonsScreen(),
    );
  }
}

class ThemedButtonsScreen extends StatelessWidget {
  const ThemedButtonsScreen({super.key});

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: const Text('Themed Buttons Screen'),
      ),
      body: Center(
        child: Column(
          mainAxisSize: MainAxisSize.min,
          children: [
            // TODO: Add primary button
            // TODO: Add secondary button
            // TODO: Add disabled button
          ],
        ),
      ),
    );
  }
}

Task 1

Task 2

Task 3

Task 4

Task 5

Solution

Flutter

import 'package:flutter/material.dart';

void main() {
  runApp(const MyApp());
}

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Theming Demo',
      theme: ThemeData(
        primaryColor: Colors.blue,
        colorScheme: ColorScheme.fromSeed(seedColor: Colors.blue).copyWith(secondary: Colors.orange),
        disabledColor: Colors.grey.shade400,
      ),
      home: const ThemedButtonsScreen(),
    );
  }
}

class ThemedButtonsScreen extends StatelessWidget {
  const ThemedButtonsScreen({super.key});

  @override
  Widget build(BuildContext context) {
    final theme = Theme.of(context);
    return Scaffold(
      appBar: AppBar(
        title: const Text('Themed Buttons Screen'),
      ),
      body: Center(
        child: Column(
          mainAxisSize: MainAxisSize.min,
          children: [
            ElevatedButton(
              onPressed: () {},
              child: const Text('Primary Button'),
            ),
            const SizedBox(height: 16),
            OutlinedButton(
              onPressed: () {},
              style: OutlinedButton.styleFrom(
                foregroundColor: theme.colorScheme.secondary,
              ),
              child: const Text('Secondary Button'),
            ),
            const SizedBox(height: 16),
            ElevatedButton(
              onPressed: null, // disabled
              style: ElevatedButton.styleFrom(
                backgroundColor: theme.disabledColor,
                foregroundColor: Colors.white,
              ),
              child: const Text('Disabled Button'),
            ),
          ],
        ),
      ),
    );
  }
}

We define a ThemeData with primary and secondary colors. The MaterialApp uses this theme, so all widgets inside can access it. The ElevatedButton uses the primary color automatically. The OutlinedButton uses the secondary color from the theme. The disabled button uses the theme's disabled color to appear faded. This way, all buttons share consistent colors and styles, making the UI look unified and professional.

Final Result

Completed Screen

-------------------------
| Themed Buttons Screen  |
|-----------------------|
| [Primary Button]      |
| [Secondary Button]    |
| [Disabled Button]     |
-------------------------

Tapping Primary Button triggers onPressed action (prints or no-op here)

Tapping Secondary Button triggers onPressed action

Disabled Button is not tappable and appears faded

Stretch Goal

Add a toggle to switch between light and dark theme to see consistent theming in both modes.

💡 Hint

Use ThemeMode and define both lightTheme and darkTheme in MaterialApp. Add a switch in the app bar to toggle ThemeMode.

Practice

1. Why is using theming in a Flutter app important for UI design?

easy

A. It makes the app run faster on older devices.

B. It automatically fixes all bugs in the app.

C. It ensures colors and fonts are consistent across all screens.

D. It removes the need to write any UI code.

Solution

1. Step 1: Understand what theming controls

   Theming sets colors, fonts, and styles globally for the app's UI.

2. Step 2: Recognize the benefit of consistency

   Consistent colors and fonts make the app look professional and unified.

3. Final Answer:

   It ensures colors and fonts are consistent across all screens. -> Option C

4. Quick Check:

   Theming = Consistent UI [OK]

Hint: Theming controls app-wide colors and fonts [OK]

Common Mistakes:

• Thinking theming fixes bugs
• Assuming theming improves app speed
• Believing theming removes UI coding

2. Which of the following is the correct way to apply a theme in Flutter?

easy

A. MaterialApp(style: ThemeData(primaryColor: Colors.blue))

B. MaterialApp(theme: ThemeData(primaryColor: Colors.blue))

C. MaterialApp(themeData: ThemeData(primaryColor: Colors.blue))

D. MaterialApp(appTheme: ThemeData(primaryColor: Colors.blue))

Solution

1. Step 1: Recall Flutter's theming syntax

   The theme is set using the theme property inside MaterialApp.

2. Step 2: Identify the correct property name

   Only theme is valid; others like style, themeData, or appTheme are incorrect.

3. Final Answer:

   MaterialApp(theme: ThemeData(primaryColor: Colors.blue)) -> Option B

4. Quick Check:

   Use theme property in MaterialApp [OK]

Hint: Use 'theme' property in MaterialApp for theming [OK]

Common Mistakes:

• Using incorrect property names like style or themeData
• Confusing theme with style
• Trying to set theme outside MaterialApp

3. Given this Flutter code snippet, what color will the AppBar background be?

MaterialApp(
  theme: ThemeData(primaryColor: Colors.red),
  home: Scaffold(
    appBar: AppBar(),
  ),
)

medium

A. Blue

B. Default grey

C. Green

D. Red

Solution

1. Step 1: Check the theme's primaryColor

   The theme sets primaryColor to Colors.red.

2. Step 2: Understand AppBar uses primaryColor

   AppBar background color defaults to the theme's primaryColor.

3. Final Answer:

   Red -> Option D

4. Quick Check:

   AppBar color = theme primaryColor = Red [OK]

Hint: AppBar uses theme's primaryColor by default [OK]

Common Mistakes:

• Assuming AppBar defaults to blue
• Ignoring the theme's primaryColor
• Thinking AppBar color is grey by default

4. What is wrong with this Flutter code if the app theme does not apply?

MaterialApp(
  themeData: ThemeData(primaryColor: Colors.green),
  home: Scaffold(
    appBar: AppBar(),
  ),
)

medium

A. The property should be 'theme', not 'themeData'.

B. ThemeData cannot set primaryColor.

C. AppBar does not use theme colors.

D. Scaffold must be inside a Theme widget.

Solution

1. Step 1: Identify the property used for theming

   The correct property in MaterialApp is theme, not themeData.

2. Step 2: Understand impact of wrong property

   Using themeData means the theme is ignored, so default styles apply.

3. Final Answer:

   The property should be 'theme', not 'themeData'. -> Option A

4. Quick Check:

   Use 'theme' property for theming [OK]

Hint: Use 'theme' property, not 'themeData' [OK]

Common Mistakes:

• Using 'themeData' instead of 'theme'
• Thinking AppBar ignores theme
• Believing Scaffold needs Theme widget

5. You want your Flutter app to have a consistent font and color scheme across all screens. Which approach best achieves this?

hard

A. Use ThemeData in MaterialApp and reference theme styles in widgets.

B. Create separate styles for each screen manually.

C. Set colors and fonts individually in each widget.

D. Apply styles only to the home screen.

Solution

1. Step 1: Understand the goal of consistent UI

   Consistency means using the same colors and fonts everywhere without repeating code.

2. Step 2: Identify the best Flutter approach

   Using ThemeData in MaterialApp sets global styles; widgets can then use theme references.

3. Step 3: Compare other options

   Setting styles individually or per screen causes inconsistency and more work.

4. Final Answer:

   Use ThemeData in MaterialApp and reference theme styles in widgets. -> Option A

5. Quick Check:

   Global ThemeData = Consistent UI [OK]

Hint: Use global ThemeData for consistent fonts and colors [OK]

Common Mistakes:

• Styling widgets individually causing inconsistency
• Creating separate styles per screen
• Styling only the home screen