Refactor theme colors and styles across the application

- Introduced AppColors class to centralize color definitions for better maintainability and consistency.
- Updated various screens (Settings, Vault Access, Note Card, etc.) to use AppColors for styling instead of hardcoded colors.
- Enhanced UI elements with improved color contrast and accessibility.
- Replaced gradient backgrounds with defined color schemes for a cohesive look.
- Refactored button styles and text colors to align with the new theme structure.

lib/app.dart

@@ -14,6 +14,7 @@ import 'package:notas/screens/biometric_gate_screen.dart';
 import 'package:notas/screens/home_screen.dart';
 import 'package:notas/screens/settings_screen.dart';
 import 'package:notas/screens/vault_access_screen.dart';
+import 'package:notas/theme/app_colors.dart';
 import 'package:notas/theme/app_theme.dart';
 import 'package:notas/widgets/sync_status.dart';
 import 'package:path/path.dart' as p;
@@ -68,7 +69,7 @@ class _NotesAppState extends State<NotesApp>
   String? _syncErrorMessage;
   int _syncOperationId = 0;
   int _homeRefreshToken = 0;
-  Color _themeSeedColor = Colors.amber;
+  Color _themeSeedColor = AppColors.defaultThemeSeedColor;
 
   @override
   void initState() {
@@ -859,15 +860,7 @@ class _NotesAppState extends State<NotesApp>
           child: Scaffold(
             body: Container(
               decoration: const BoxDecoration(
-                gradient: LinearGradient(
-                  colors: [
-                    Color(0xFF191A1D),
-                    Color(0xFF222326),
-                    Color(0xFF101114),
-                  ],
-                  begin: Alignment.topLeft,
-                  end: Alignment.bottomRight,
-                ),
+                gradient: AppColors.backdropGradient,
               ),
               child: SafeArea(
                 child: Column(