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/theme/app_theme.dart

@@ -1,15 +1,17 @@
 import 'package:flutter/material.dart';
+import 'package:notas/theme/app_colors.dart';
 
 class AppTheme {
   static ThemeData theme({Color seedColor = Colors.amber}) {
     final Brightness foregroundBrightness =
         ThemeData.estimateBrightnessForColor(seedColor);
-    final Color foregroundColor =
-        foregroundBrightness == Brightness.dark ? Colors.white : Colors.black;
+    final Color foregroundColor = foregroundBrightness == Brightness.dark
+        ? AppColors.textPrimary
+        : AppColors.textOnAccent;
 
     return ThemeData(
       useMaterial3: true,
-      scaffoldBackgroundColor: const Color.fromRGBO(31, 32, 33, 1),
+      scaffoldBackgroundColor: AppColors.scaffoldBackground,
       colorScheme: ColorScheme.fromSeed(
         seedColor: seedColor,
         brightness: Brightness.dark,
@@ -20,4 +22,4 @@ class AppTheme {
       ),
     );
   }
-}
+}