added basic app theme

lib/app_theme.dart

@@ -0,0 +1,37 @@
+import 'package:flutter/material.dart';
+
+class AppTheme {
+  AppTheme._();
+
+  static ThemeData get lightTheme => _baseTheme(
+    colorScheme: ColorScheme.fromSeed(
+      seedColor: Colors.indigo,
+      brightness: Brightness.light,
+    ),
+  );
+
+  static ThemeData get darkTheme => _baseTheme(
+    colorScheme: ColorScheme.fromSeed(
+      seedColor: Colors.indigo,
+      brightness: Brightness.dark,
+    ),
+  );
+
+  static ThemeData _baseTheme({required ColorScheme colorScheme}) {
+    final theme = ThemeData(useMaterial3: true, colorScheme: colorScheme);
+    final universalBorderRadius = BorderRadius.circular(12);
+
+    return theme.copyWith(
+      inputDecorationTheme: InputDecorationTheme(
+        border: OutlineInputBorder(borderRadius: universalBorderRadius),
+      ),
+
+      listTileTheme: ListTileThemeData(
+        shape: RoundedRectangleBorder(
+          borderRadius: universalBorderRadius,
+          side: BorderSide(color: colorScheme.secondaryContainer, width: 2),
+        ),
+      ),
+    );
+  }
+}

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(),