Enhance HTMLLoader to include CSS files in error templates and add error handling for missing CSS. Define APP_DOMAIN in main.php and update error template structure.

public/main.php

@@ -9,6 +9,7 @@ class gb {
         }
 
         // Define Paths
+        define("APP_DOMAIN", $_SERVER["SERVER_NAME"]);
         define("APP_PATH", __DIR__ . "/..");
         define("FRONTEND_PATH", __DIR__ . "/");
         define("TEMPLATE_PATH", FRONTEND_PATH . "templates/");