how to add animation for theme switching in flutter?

It’s not hard, but you need to do several things.

1. You need to create your own theme styles. I’ve used inherited widget to do it. (If you change ThemeData widget it will animate the change, and we don’t need it, that’s why I’m saving Colors in another class)
2. Find the button (or in my case switcher) coordinates.
3. Run animation.

update! I've converted our code to a package with simple api.

import 'package:equatable/equatable.dart';
import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return BrandTheme(
      child: Builder(builder: (context) {
        return MaterialApp(
          title: 'Flutter Demo',
          theme: BrandTheme.of(context).themeData,
          home: MyHomePage(),
        );
      }),
    );
  }
}

GlobalKey switherGlobalKey = GlobalKey();

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key}) : super(key: key);

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage>
    with SingleTickerProviderStateMixin {
  AnimationController _controller;

  @override
  void initState() {
    _controller = AnimationController(
      duration: const Duration(milliseconds: 300),
      vsync: this,
    );

    _controller.forward();
    super.initState();
  }

  @override
  void dispose() {
    _controller.dispose();

    super.dispose();
  }

  int _counter = 0;
  BrandThemeModel oldTheme;
  Offset switcherOffset;

  void _incrementCounter() {
    setState(() {
      _counter++;
    });
  }

  _getPage(brandTheme, {isFirst = false}) {
    return Scaffold(
      backgroundColor: brandTheme.color2,
      appBar: AppBar(
        backgroundColor: brandTheme.color1,
        title: Text(
          'Flutter Demo Home Page',
          style: TextStyle(color: brandTheme.textColor2),
        ),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.spaceAround,
          children: <Widget>[
            Text(
              'You have pushed the button this many times:',
              style: TextStyle(
                color: brandTheme.textColor1,
              ),
            ),
            Text(
              '$_counter',
              style: TextStyle(color: brandTheme.textColor1, fontSize: 200),
            ),
            Switch(
              key: isFirst ? switherGlobalKey : null,
              onChanged: (needDark) {
                oldTheme = brandTheme;
                BrandTheme.instanceOf(context).changeTheme(
                  needDark ? BrandThemeKey.dark : BrandThemeKey.light,
                );
              },
              value: BrandTheme.of(context).brightness == Brightness.dark,
            )
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _incrementCounter,
        tooltip: 'Increment',
        child: Icon(
          Icons.add,
        ),
      ),
    );
  }

  @override
  void didUpdateWidget(Widget oldWidget) {
    var theme = BrandTheme.of(context);
    if (theme != oldTheme) {
      _getSwitcherCoodinates();
      _controller.reset();
      _controller.forward().then(
        (_) {
          oldTheme = theme;
        },
      );
    }
    super.didUpdateWidget(oldWidget);
  }

  void _getSwitcherCoodinates() {
    RenderBox renderObject = switherGlobalKey.currentContext.findRenderObject();
    switcherOffset = renderObject.localToGlobal(Offset.zero);
  }

  @override
  Widget build(BuildContext context) {
    var brandTheme = BrandTheme.of(context);

    if (oldTheme == null) {
      return _getPage(brandTheme, isFirst: true);
    }
    return Stack(
      children: <Widget>[
        if(oldTheme != null) _getPage(oldTheme),
        AnimatedBuilder(
          animation: _controller,
          child: _getPage(brandTheme, isFirst: true),
          builder: (_, child) {
            return ClipPath(
              clipper: MyClipper(
                sizeRate: _controller.value,
                offset: switcherOffset.translate(30, 15),
              ),
              child: child,
            );
          },
        ),
      ],
    );
  }
}

class MyClipper extends CustomClipper<Path> {
  MyClipper({this.sizeRate, this.offset});
  final double sizeRate;
  final Offset offset;

  @override
  Path getClip(Size size) {
    var path = Path()
      ..addOval(
        Rect.fromCircle(center: offset, radius: size.height * sizeRate),
      );

    return path;
  }

  @override
  bool shouldReclip(CustomClipper<Path> oldClipper) => true;
}

class BrandTheme extends StatefulWidget {
  final Widget child;

  BrandTheme({
    Key key,
    @required this.child,
  }) : super(key: key);

  @override
  BrandThemeState createState() => BrandThemeState();

  static BrandThemeModel of(BuildContext context) {
    final inherited =
        (context.dependOnInheritedWidgetOfExactType<_InheritedBrandTheme>());
    return inherited.data.brandTheme;
  }

  static BrandThemeState instanceOf(BuildContext context) {
    final inherited =
        (context.dependOnInheritedWidgetOfExactType<_InheritedBrandTheme>());
    return inherited.data;
  }
}

class BrandThemeState extends State<BrandTheme> {
  BrandThemeModel _brandTheme;

  BrandThemeModel get brandTheme => _brandTheme;

  @override
  void initState() {
    final isPlatformDark =
        WidgetsBinding.instance.window.platformBrightness == Brightness.dark;
    final themeKey = isPlatformDark ? BrandThemeKey.dark : BrandThemeKey.light;
    _brandTheme = BrandThemes.getThemeFromKey(themeKey);
    super.initState();
  }

  void changeTheme(BrandThemeKey themeKey) {
    setState(() {
      _brandTheme = BrandThemes.getThemeFromKey(themeKey);
    });
  }

  @override
  Widget build(BuildContext context) {
    return _InheritedBrandTheme(
      data: this,
      child: widget.child,
    );
  }
}

class _InheritedBrandTheme extends InheritedWidget {
  final BrandThemeState data;

  _InheritedBrandTheme({
    this.data,
    Key key,
    @required Widget child,
  }) : super(key: key, child: child);

  @override
  bool updateShouldNotify(_InheritedBrandTheme oldWidget) {
    return true;
  }
}

ThemeData defaultThemeData = ThemeData(
  floatingActionButtonTheme: FloatingActionButtonThemeData(
    shape: RoundedRectangleBorder(),
  ),
);

class BrandThemeModel extends Equatable {
  final Color color1;
  final Color color2;

  final Color textColor1;
  final Color textColor2;
  final ThemeData themeData;
  final Brightness brightness;

  BrandThemeModel({
    @required this.color1,
    @required this.color2,
    @required this.textColor1,
    @required this.textColor2,
    @required this.brightness,
  }) : themeData = defaultThemeData.copyWith(brightness: brightness);

  @override
  List<Object> get props => [
        color1,
        color2,
        textColor1,
        textColor2,
        themeData,
        brightness,
      ];
}

enum BrandThemeKey { light, dark }

class BrandThemes {
  static BrandThemeModel getThemeFromKey(BrandThemeKey themeKey) {
    switch (themeKey) {
      case BrandThemeKey.light:
        return lightBrandTheme;
      case BrandThemeKey.dark:
        return darkBrandTheme;
      default:
        return lightBrandTheme;
    }
  }
}

BrandThemeModel lightBrandTheme = BrandThemeModel(
  brightness: Brightness.light,
  color1: Colors.blue,
  color2: Colors.white,
  textColor1: Colors.black,
  textColor2: Colors.white,
);

BrandThemeModel darkBrandTheme = BrandThemeModel(
  brightness: Brightness.dark,
  color1: Colors.red,
  color2: Colors.black,
  textColor1: Colors.blue,
  textColor2: Colors.yellow,
);

class ThemeRoute extends PageRouteBuilder {
  ThemeRoute(this.widget)
      : super(
          pageBuilder: (
            context,
            animation,
            secondaryAnimation,
          ) =>
              widget,
          transitionsBuilder: transitionsBuilder,
        );

  final Widget widget;
}

Widget transitionsBuilder(
  BuildContext context,
  Animation<double> animation,
  Animation<double> secondaryAnimation,
  Widget child,
) {
  var _animation = Tween<double>(
    begin: 0,
    end: 100,
  ).animate(animation);
  return SlideTransition(
    position: Tween<Offset>(
      begin: const Offset(0, 1),
      end: Offset.zero,
    ).animate(animation),
    child: Container(
      child: child,
    ),
  );
}