added basic app theme

lib/main.dart

@@ -1,5 +1,6 @@
 import 'package:flutter/material.dart';
 
+import 'app_theme.dart';
 import 'model/repositories/local_repository.dart';
 import 'pages/task_edit_page.dart';
 import 'pages/task_overview_page.dart';
@@ -33,6 +34,8 @@ class MainApp extends StatelessWidget {
   @override
   Widget build(BuildContext context) {
     return MaterialApp(
+      theme: AppTheme.lightTheme,
+      darkTheme: AppTheme.darkTheme,
       routes: {
         TaskOverviewPage.routeName: (context) => TaskOverviewPage(),
         TaskEditPage.routeName: (context) => TaskEditPage(),