[compass_app] Mark classes used for namespacing as abstract final (#2540)

As they are just used for namespacing static properties, these classes
shouldn't be extended, implemented, or instantiated.

compass_app/app/lib/ui/core/themes/colors.dart

@@ -4,7 +4,7 @@
 
 import 'package:flutter/material.dart';
 
-class AppColors {
+abstract final class AppColors {
   static const black1 = Color(0xFF101010);
   static const white1 = Color(0xFFFFF7FA);
   static const grey1 = Color(0xFFF2F2F2);

compass_app/app/lib/ui/core/themes/theme.dart

@@ -6,7 +6,7 @@ import 'colors.dart';
 import '../ui/tag_chip.dart';
 import 'package:flutter/material.dart';
 
-class AppTheme {
+abstract final class AppTheme {
   static const _textTheme = TextTheme(
     headlineLarge: TextStyle(
       fontSize: 32,