[Flutter] Katana Theme

Hello. My name is Masaru Hirose.

This time, I maintained the theme and design area.

I have created a package:

Allows for systematic management of colors, text styles, etc.

while automatically generating assets and fonts like flutter_gen.

I’ve put together some instructions on how to use it, so if you’re interested, go ahead and give it a try!


Base package for providing simple and straightforward theme definitions on Flutter.
Building system for katana theme packages. Create a list of image assets from the Asset folder.


Flutter's handling of themes is complex and monstrous.

In Flutter, themes for apps are compiled in ThemeData, but there are various themes in ThemeData, and it is very difficult to know which theme exists in the first place, and where in the widget will change if you change which theme.

Flutter is basically based on MaterialDesign.

For this reason, I have created a package that allows you to simply define your design using color schemes and text sizes for MaterialDesign.

In addition, I have introduced a mechanism to automatically generate code from the file structure of assets and fonts used in flutter_gen, etc., so that code can be managed systematically.

The following features are available

  • Specify colors with ColorScheme in MaterialDesing3
  • Specify text size with TypeScale in MaterialDesign3
  • Ability to apply the above to ThemeData and specify it to the application.
  • Automatic generation of code to retrieve the path to the image and ImageProvider from the assets folder
  • Automatic generation of code that allows specification of font families from the fonts folder
  • Specify gradient color from theme
  • Other extensions for simple design conversion

It can be listed as follows.

final theme = AppTheme(
  primary: Colors.blue,
  secondary: Colors.cyan,

  "test text",
  style: theme.text.bodyMedium.withColor(theme.color.primary),


Import the following package for code generation using build_runner.

flutter pub add katana_theme
flutter pub add --dev build_runner
flutter pub add --dev katana_theme_builder


Theme Creation

First, create AppThemeData with @appTheme annotation.

Please add part 'original filename.theme.dart' to the file that uses @appTheme.

// theme.dart

part 'theme.theme.dart';

final theme = AppThemeData();

Creation of AppThemeScope and application of the theme to MaterialApp

Create an AppThemeScope on top of MaterialApp, etc. and pass the AppThemeData defined earlier.

By passing theme.toThemeData() to MaterialApp's theme, you can apply the theme defined in AppThemeData to your app.

// main.dart

  theme: theme,
  child: MaterialApp(
    home: const MyHomePage(
      title: "Flutter Demo",
    title: "Flutter Demo",
    theme: theme.toThemeData(),

Asset Definition

Edit pubspec.yaml with reference to the official site below so that the assets can be loaded in the application.

How to use images (and other assets) in your Flutter app.
// pubspec.yaml

    - assets/images/

Font Definition

Edit pubspec.yaml with reference to the official site below so that the fonts can be loaded in the application.

How to use custom fonts.
// pubspec.yaml

    - family: RobotoMono
        - asset: fonts/RobotoMono-Regular.ttf
        - asset: fonts/RobotoMono-Bold.ttf
          weight: 700

Code Generation

Automatic code generation is performed by entering the following command.

flutter pub run build_runner build --delete-conflicting-outputs

Theme Designation

You can specify a theme in AppThemeData.

The following colors can be specified according to the Material Design color scheme.

Build beautiful, usable products faster. Material Design is an adaptable system—backed by open-source code—that helps teams build high quality digital experiences.
  • primary
  • secondary
  • tertiary
  • primaryContainer
  • secondaryContainer
  • tertiaryContainer
  • disabled
  • outline
  • error
  • surface
  • background
  • onPrimary
  • onSecondary
  • onTertiary
  • onPrimaryContainer
  • onSecondaryContainer
  • onTertiaryContainer
  • onDisabled
  • onSurface
  • onBackground
  • onError

In addition, the following additional colors can be set

  • weak
    • Light letter colors. Used for colors you don't want to stand out too much.
  • warning
    • Color of caution.
  • info
    • Colors for displaying small bits of information.
  • success
    • Color of success.
  • onWeak
    • Text color when weak is the background color.
  • onInfo
    • Text color when info is the background color.
  • onSuccess
    • Text color when success is used as the background color.
  • onWarning
    • Text color when warning is the background color.
  • splashColor
    • The effect color when the button is tapped.
  • shadow
    • Shadow color.
  • inverseSurface
    • Inverted color of surface.
  • onInverseSurface
    • Text color when inverseSurface is the background color.

The following TypeScale can be specified according to the Typography of Material Design.

The Material Design type scale includes a range of contrasting styles that support the needs of your product and its content.
  • displayLarge
  • displayMedium
  • displaySmall
  • headlineLarge
  • headlineMedium
  • headlineSmall
  • titleLarge
  • titleMedium
  • titleSmall
  • bodyLarge
  • bodyMedium
  • bodySmall
  • labelLarge
  • labelMedium
  • labelSmall

How to use

AppThemeData is defined globally and can be referenced from anywhere.

The following themes can be obtained from AppThemeData.

  • color
    • ColorScheme defined when creating AppThemeData.
  • text
    • TypeScale defined when creating AppThemeData.
  • asset
    • Assets under the assets folder created in code generation
  • font
    • FontFamily created by Code Generation.
  • widget
    • Fixed wedget definition, such as indicator.
    • This is defined in the theme extension described below.
class TestPage extends StatelessWidget {
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text("Title"), backgroundColor: theme.color.secondary),
      body: Column(
        crossAxisAlignment: CrossAxisAlignment.start,
          Center(child: CircleAvatar(backgroundImage: theme.asset.userIcon.provider)),
          Text("User Name", style: theme.text.displayMedium)

Theme Extension

Some applications may use special colors or font sizes.

If you want to specify a single location, you can specify it directly in the widget, but if it spans multiple locations within the app, it is more efficient to specify it in the theme.

This package adds a new theme by using extension.

Extensions can be created by specifying the following classes on.

  • ColorThemeData
    • Add color.
  • TextThemeData
    • Add TextStyle, such as font size.
  • AssetThemeData
    • Add assets such as images.
  • WidgetThemeData
    • Add indicators and other wedgets.
extension ColorThemeDataExtensions on ColorThemeData {
  Color get myColor => Colors.red;

extension WidgetThemeDataExtensions on WidgetThemeData {
  Widget get indicator => const LinearProgressIndicator();

Additional usage


Gradients can be applied by specifying GradientColor to the theme.

GradientColor is recognized as a normal color if nothing is done, but can be used as a gradient color with toLinearGradient().

// theme.dart

final theme = AppThemeData(
  primary: GradientColor(
    Colors.red,  // This is recognized as the normal color.

// test_page.dart

class TestPage extends StatelessWidget {
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text("Title")),
      body: Container(
        decoration: BoxDecoration(
          gradient: theme.color.primary.toLinearGradient(),

Conversion Methods

The data of the theme can be slightly modified and output for actual use.

final Color darkenColor = theme.color.primary.darken(); // Output a little darker color
final Color lightenColor = theme.color.primary.lighten(); // Output with a little brighter color

final TextStyle smallizeText = theme.text.bodyMedium.smallize(); // Output with a slightly smaller font size
final TextStyle largizeText = theme.text.bodyMedium.largize(); // Output with a slightly larger font size
final TextStyle changedFontSizeText = theme.text.bodyMedium.withSize(18); // Output with font size 18

final TextStyle darkenText = theme.text.bodyMedium.darken(); // Output with slightly darker text color
final TextStyle lightenText = theme.text.bodyMedium.lighten(); // Output with a little brighter text color
final TextStyle changedFontColorText = theme.text.bodyMedium.withColor(theme.color.primary); // Output a little text color as primary color

final TextStyle changedOpacityText = theme.text.bodyMedium.withOpacity(0.5); // Output text with half transparency
final TextStyle changedFontWeightText = theme.text.bodyMedium.withBold(); // Bold text output


I made it for my own use, but if you think it fits your implementation philosophy, by all means, use it!

Also, I releasing the source here, so issues and PullRequests are welcome!

If you have any further work requests, please contact me directly through my Twitter or website!

Offers app development and apps using Flutter and Unity. Includes information on music and videos created by the company. Distribution of images and video materials. We also accept orders for work.

GitHub Sponsors

Sponsors are always welcome. Thank you for your support!

Developed the katana/masamune framework, which has dramatically improved the overall efficiency of Flutter-based application development.