basic structure with material 3

lib/myApp.dart

@@ -6,8 +6,12 @@ class MyApp extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
-    return const MaterialApp(
+    return MaterialApp(
       title: 'My App',
+      theme: ThemeData(
+        useMaterial3: true,
+        primaryColor: Colors.red,
+      ),
       home: Navigation()
     );
   }

lib/navigation.dart

@@ -1,6 +1,7 @@
 import 'dart:ffi';
 
 import 'package:flutter/material.dart';
+import 'testUI.dart';
 
 class Navigation extends StatefulWidget {
   const Navigation({super.key});
@@ -11,9 +12,9 @@ class Navigation extends StatefulWidget {
 
 class _NavigationState extends State<Navigation> {
 
-  int _selectedIndex = 1;
+  int _selectedIndex = 0;
   static const List<Widget> _widgetOptions = <Widget>[
-    Text("Calendar"),
+    TestUI(),
     Text("Inbox"),
     Text("Notes"),
   ];

lib/testUI.dart

@@ -0,0 +1,29 @@
+import 'package:flutter/material.dart';
+
+class TestUI extends StatefulWidget {
+  const TestUI({super.key});
+
+  @override
+  State<TestUI> createState() => _TestUIState();
+}
+
+class _TestUIState extends State<TestUI> {
+  int _test = 0;
+
+  void _onPressed() {
+    setState(() {
+      _test = 1;
+    });
+  }
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      body: Center(
+        child: Column(children: [
+          FloatingActionButton(onPressed: _onPressed),
+          Text('$_test'),
+        ],),
+      ),
+    );
+  }
+}