A simple way to handle light/dark theme with animation tied to ThemeData.

theme_brightness_animated_builder.dart

import 'package:flutter/material.dart';

/// ```dart
/// class MyLightAndDarkThemeWidget extends StatelessWidget {
///   @override
///   Widget build(BuildContext context) {
///     return ThemeBrightnessAnimatedBuilder(
///       builder: (context, t, child) {
///         final lightThemeColor = Colors.red.shade800;
///         final darkThemeColor = Colors.red.shade200;
///
///         return Container(
///           /// Light theme color goes on the right, dark theme on the left.
///           color: Color.lerp(lightThemeColor, darkThemeColor, t),
///         );
///       },
///     );
///   }
/// }
/// ```
class ThemeBrightnessAnimatedBuilder extends StatelessWidget {
  /// A builder that automatically transitions t from `0.0` to `1.0` depending on if
  /// [ThemeData.brightness] is [Brightness.dark] or [Brightness.light]
  const ThemeBrightnessAnimatedBuilder({
    Key key,
    @required this.builder,
    this.child,
  }) : super(key: key);

  /// `0.0` when [ThemeData.brightness] is [Brightness.light]
  /// `1.0` when [ThemeData.brightness] is [Brightness.dark]
  final Widget Function(BuildContext, double, Widget) builder;

  /// A child to pass through without rebuilding when animating.
  final Widget child;

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

    return TweenAnimationBuilder(
      tween: Tween<double>(end: theme.brightness == Brightness.light ? 0 : 1),
      curve: Curves.linear,
      duration: kThemeAnimationDuration,
      builder: builder,
      child: child,
    );
  }
}