All 7 App Theming & Dark Mode questions
More in App Theming & Dark Mode
What is a ThemeData object in Flutter, and how does it let you define consistent colors, fonts, and styling across an entire application from a single central place? How does Flutter's Material 3 ColorScheme let you define a comprehensive, cohesive set of colors for your app derived from just a small number of key seed colors? How can you let a user manually select and persist their own preferred theme, such as light, dark, or system default, rather than only relying on the device's automatic system setting? How do custom ThemeExtensions in Flutter let you add your own additional custom design tokens, beyond what the built in ThemeData already provides, to your app's central theme? How should a large Flutter application with a substantial design system organize its theming code to keep it maintainable as the number of screens and custom components grows significantly? How can you smoothly animate the transition between light and dark theme when a user toggles the setting, rather than having the entire app's colors change instantly and abruptly?
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Flutter › App Theming & Dark Mode

How do you implement dark mode support in a Flutter app, letting the app automatically switch its appearance based on the device's system wide dark mode setting?

Beginner

Answer

Flutter lets you define two separate ThemeData objects, one representing your app's light mode appearance and another representing its dark mode appearance, and by providing both to your MaterialApp widget through the theme and darkTheme properties, along with setting themeMode to system, your app will automatically switch between the two based on whatever dark or light mode setting the user has currently selected at the operating system level, without requiring any additional custom logic to detect that system setting yourself.
MaterialApp(
  theme: ThemeData.light(),
  darkTheme: ThemeData.dark(),
  themeMode: ThemeMode.system,
)
Real-world example A news reading app automatically switches to a comfortable dark theme in the evening when a user has enabled dark mode at the system level on their phone, without the user needing to manually toggle any setting within the app itself.

Common follow-ups: How do you let a user manually override the system theme setting from within your own app?;What Flutter widget lets you detect whether the app is currently displaying in dark mode?

Responsive & Adaptive UI Design;State Management with Provider
All 7 App Theming & Dark Mode questions