Refactor helper drift in lib: centralize theme/translation and remove app() hotspots

lib/Service/Auth/AuthSettingsGateway.php

@@ -49,6 +49,16 @@ class AuthSettingsGateway
         return (string) $this->settingsAppGateway->getAppTheme();
     }
 
+    public function defaultTheme(): string
+    {
+        return $this->settingsAppGateway->resolveDefaultTheme();
+    }
+
+    public function normalizeTheme(?string $theme): string
+    {
+        return $this->settingsAppGateway->normalizeTheme($theme);
+    }
+
     public function getDefaultRoleId(): int
     {
         return (int) $this->settingsDefaultsGateway->getDefaultRoleId();