Flutter ThemeExtension for custom app colors.

app_colors_extension.dart

import 'package:flutter/material.dart';

/// `ThemeExtension` template for custom colors.
///
/// For example purposes, it has all required fields from the default Material `ColorScheme`.
/// But you can add, rename and delete any fields your need.
///
/// ### Motivation
///
/// At the beginning, you may not know if your colors will fit into the Material `ColorScheme`,
/// but you still decided to start using `ColorScheme`, and only then realize that you need additional fields.
/// You will create `ThemeExtension` for only the additional fields, and as the result, you will have your colors
/// scattered between the `ColorScheme` and `ThemeExtension` with a few extra colors.
///
/// With this template, you can collect all fields in one place,
/// and don't worry about future changes to the Material or your design.
///
/// Or you can just quickly copy-paste this file and rename fields to your needs.
class AppColorsExtension extends ThemeExtension<AppColorsExtension> {
  AppColorsExtension({
    required this.primary,
    required this.onPrimary,
    required this.secondary,
    required this.onSecondary,
    required this.error,
    required this.onError,
    required this.background,
    required this.onBackground,
    required this.surface,
    required this.onSurface,
  });

  final Color primary;
  final Color onPrimary;
  final Color secondary;
  final Color onSecondary;
  final Color error;
  final Color onError;
  final Color background;
  final Color onBackground;
  final Color surface;
  final Color onSurface;

  @override
  ThemeExtension<AppColorsExtension> copyWith({
    Color? primary,
    Color? onPrimary,
    Color? secondary,
    Color? onSecondary,
    Color? error,
    Color? onError,
    Color? background,
    Color? onBackground,
    Color? surface,
    Color? onSurface,
  }) {
    return AppColorsExtension(
      primary: primary ?? this.primary,
      onPrimary: onPrimary ?? this.onPrimary,
      secondary: secondary ?? this.secondary,
      onSecondary: onSecondary ?? this.onSecondary,
      error: error ?? this.error,
      onError: onError ?? this.onError,
      background: background ?? this.background,
      onBackground: onBackground ?? this.onBackground,
      surface: surface ?? this.surface,
      onSurface: onSurface ?? this.onSurface,
    );
  }

  @override
  ThemeExtension<AppColorsExtension> lerp(
    covariant ThemeExtension<AppColorsExtension>? other,
    double t,
  ) {
    if (other is! AppColorsExtension) {
      return this;
    }

    return AppColorsExtension(
      primary: Color.lerp(primary, other.primary, t)!,
      onPrimary: Color.lerp(onPrimary, other.onPrimary, t)!,
      secondary: Color.lerp(secondary, other.secondary, t)!,
      onSecondary: Color.lerp(onSecondary, other.onSecondary, t)!,
      error: Color.lerp(error, other.error, t)!,
      onError: Color.lerp(onError, other.onError, t)!,
      background: Color.lerp(background, other.background, t)!,
      onBackground: Color.lerp(onBackground, other.onBackground, t)!,
      surface: Color.lerp(surface, other.surface, t)!,
      onSurface: Color.lerp(onSurface, other.onSurface, t)!,
    );
  }
}

/// Optional. If you also want to assign colors in the `ColorScheme`.
extension ColorSchemeBuilder on AppColorsExtension {
  ColorScheme toColorScheme(Brightness brightness) {
    return ColorScheme(
      brightness: brightness,
      primary: primary,
      onPrimary: onPrimary,
      secondary: secondary,
      onSecondary: onSecondary,
      error: error,
      onError: onError,
      background: background,
      onBackground: onBackground,
      surface: surface,
      onSurface: onSurface,
    );
  }
}

Use case?