【iOS】Fix Alert userInterfaceStyle having no effect (#39346)

Summary:
Fixes #38852

## Changelog:

[IOS] [FIXED] - Fix Alert userInterfaceStyle having no effect

Pull Request resolved: #39346

Test Plan: Run the demo from #38852 .

Reviewed By: cipolleschi

Differential Revision: D49100720

Pulled By: dmytrorykun

fbshipit-source-id: cf8213dafc840720e9d74626e147804ea129cdf4

packages/react-native/React/CoreModules/RCTAlertController.mm

@@ -43,9 +43,13 @@ - (UIWindow *)alertWindow
 
 - (void)show:(BOOL)animated completion:(void (^)(void))completion
 {
-  UIUserInterfaceStyle style = RCTSharedApplication().delegate.window.overrideUserInterfaceStyle
-      ? RCTSharedApplication().delegate.window.overrideUserInterfaceStyle
-      : UIUserInterfaceStyleUnspecified;
+  UIUserInterfaceStyle style = self.overrideUserInterfaceStyle;
+  if (style == UIUserInterfaceStyleUnspecified) {
+    style = RCTSharedApplication().delegate.window.overrideUserInterfaceStyle
+        ? RCTSharedApplication().delegate.window.overrideUserInterfaceStyle
+        : UIUserInterfaceStyleUnspecified;
+  }
+
   self.overrideUserInterfaceStyle = style;
 
   [self.alertWindow makeKeyAndVisible];