feature：add theme_status

lib/main.dart

@@ -1,9 +1,11 @@
 import 'package:flutter/material.dart';
 import 'package:fluro/fluro.dart';
+import 'package:flutter_picgo/model/theme_state.dart';
 import 'package:flutter_picgo/resources/theme_colors.dart';
 import 'package:flutter_picgo/routers/application.dart';
 import 'package:flutter_picgo/routers/routers.dart';
 import 'package:flutter_picgo/utils/db_provider.dart';
+import 'package:provider/provider.dart';
 
 void main() async {
   WidgetsFlutterBinding.ensureInitialized();
@@ -12,22 +14,46 @@ void main() async {
   runApp(App());
 }
 
-class App extends StatelessWidget {
+class App extends StatefulWidget {
   App() {
     final router = new Router();
     Routes.configureRoutes(router);
     Application.router = router;
   }
 
+  _AppState createState() => _AppState();
+}
+
+class _AppState extends State<App> {
   // This widget is the root of your application.
   @override
   Widget build(BuildContext context) {
-    return MaterialApp(
-      debugShowCheckedModeBanner: false,
-      theme: lightThemeData,
-      darkTheme: darkThemeData,
-      initialRoute: '/',
-      onGenerateRoute: Application.router.generator,
+    return MultiProvider(
+      providers: [
+        ChangeNotifierProvider(
+          create: (context) => ThemeState(),
+        ),
+      ],
+      child: Consumer<ThemeState>(
+        builder: (context, state, child) {
+          return state.currentMode == ThemeMode.system
+              ? MaterialApp(
+                  debugShowCheckedModeBanner: false,
+                  theme: lightThemeData,
+                  darkTheme: darkThemeData,
+                  initialRoute: '/',
+                  onGenerateRoute: Application.router.generator,
+                )
+              : MaterialApp(
+                  debugShowCheckedModeBanner: false,
+                  theme: state.currentMode == ThemeMode.light
+                      ? lightThemeData
+                      : darkThemeData,
+                  initialRoute: '/',
+                  onGenerateRoute: Application.router.generator,
+                );
+        },
+      ),
     );
   }
 }