Theming

Material 3 theming best practices for Flutter applications using ThemeData as the single source of truth for colors, typography, component styles, and spacing.

Core Standards

Apply these standards to ALL theming work:

• Use ThemeData as the single source of truth — never inline colors or text styles in widgets
• Reference colors via Theme.of(context).colorScheme — never Colors.blue, Colors.red, or any hardcoded Color values
• Reference text styles via Theme.of(context).textTheme — never inline TextStyle(...) in widget code
• Use ColorScheme for all color definitions — Material 3's structured color system
• Centralize component themes in ThemeData — define FilledButtonThemeData, InputDecorationTheme, etc. in the theme, not per-widget
• Define a spacing system with a base unit — no arbitrary pixel values for padding, margins, or gaps
• Support light and dark themes from the start — use ThemeData so theme switching requires zero conditional logic in widgets
• Avoid conditional logic for theming in UI — never check brightness in widget code; let ThemeData handle it
• Prefer EdgeInsets.only and EdgeInsets.symmetric — never EdgeInsets.fromLTRB (positional arguments are error-prone)

Color System

Custom Colors Class

Centralize all color definitions in a dedicated class:

abstract class AppColors {
  static const primaryColor = Color(0xFF4F46E5);
  static const secondaryColor = Color(0xFF9C27B0);
  static const errorColor = Color(0xFFDC2626);
  static const surfaceColor = Color(0xFFFAFAFA);
}

ColorScheme Configuration

The ColorScheme class includes 45 colors based on Material 3 specifications. Configure it within ThemeData:

ThemeData(
  colorScheme: ColorScheme(
    brightness: Brightness.light,
    primary: AppColors.primaryColor,
    secondary: AppColors.secondaryColor,
    error: AppColors.errorColor,
    surface: AppColors.surfaceColor,
    onPrimary: Colors.white,
    onSecondary: Colors.white,
    onError: Colors.white,
    onSurface: Colors.black,
  ),
)

For quick prototyping, use ColorScheme.fromSeed():

ThemeData(
  colorScheme: ColorScheme.fromSeed(
    seedColor: AppColors.primaryColor,
  ),
)

Light and Dark Theme Variants

class AppTheme {
  static ThemeData get light => ThemeData(
    colorScheme: ColorScheme(
      brightness: Brightness.light,
      primary: AppColors.primaryColor,
      surface: AppColors.surfaceColor,
      // ... remaining color roles
    ),
  );

  static ThemeData get dark => ThemeData(
    colorScheme: ColorScheme(
      brightness: Brightness.dark,
      primary: AppColors.primaryColorDark,
      surface: AppColors.surfaceColorDark,
      // ... remaining color roles
    ),
  );
}

Accessing Colors

@override
Widget build(BuildContext context) {
  final colorScheme = Theme.of(context).colorScheme;

  return ColoredBox(
    color: colorScheme.surface,
    child: Text(
      'Hello',
      style: TextStyle(color: colorScheme.onSurface),
    ),
  );
}

Typography

Define an AppTextStyle class with a base style and named variants (displayLarge, headlineMedium, bodyLarge, etc.), then integrate them into ThemeData.textTheme. Access styles via Theme.of(context).textTheme.

See references/typography.md for font asset setup, the full AppTextStyle class, TextTheme integration, and widget access patterns.

Component Themes

Define component themes centrally in ThemeData (e.g., filledButtonTheme, inputDecorationTheme, appBarTheme) instead of styling individual widget instances. A complete AppTheme class assembles ColorScheme, TextTheme, and all component themes into a single ThemeData.

See references/components.md for FilledButton, InputDecoration, and AppBar theme examples, the complete theme assembly, and widget access patterns.

Spacing System

Define an AppSpacing class with a base unit (e.g., 16px) and named constants (xxs through xxlg). Use EdgeInsets.only or EdgeInsets.symmetric — never EdgeInsets.fromLTRB.

See references/spacing.md for the full AppSpacing class, usage examples, and EdgeInsets preferences.

Common Patterns

Creating a Theme

1. Define AppColors with all color constants
2. Define AppTextStyle with all text style constants
3. Define AppSpacing with spacing scale based on a base unit
4. Create AppTheme class with light and dark getters
5. Configure ColorScheme, TextTheme, and component themes in each ThemeData
6. Pass AppTheme.light and AppTheme.dark to MaterialApp

Adding a New Color Token

1. Add the color constant to AppColors
2. Map it to the appropriate ColorScheme role (or create a theme extension for custom tokens)
3. Reference it via Theme.of(context).colorScheme.<role> in widgets

Dark Mode Support

1. Create separate ColorScheme instances for light and dark
2. Use the same TextTheme and component themes (they adapt automatically via colorScheme)
3. Pass both themes to MaterialApp via theme and darkTheme
4. Never check Brightness in widget code — let ThemeData handle the switch

Quick Reference

ThemeData Property	Purpose
colorScheme	Material 3 color system (45 color roles)
textTheme	Typography scale (display, headline, body…)
filledButtonTheme	FilledButton default style
inputDecorationTheme	TextField/TextFormField decoration defaults
appBarTheme	AppBar default styling
cardTheme	Card default styling
dialogTheme	Dialog default styling

Material 3 Color Role	Typical Use
primary	Key UI elements, FAB, active states
onPrimary	Text/icons on primary color
secondary	Less prominent UI elements
surface	Card, sheet, dialog backgrounds
onSurface	Text/icons on surface color
error	Error indicators, destructive actions
outline	Borders, dividers