How do I change the appearance of the navigation bar of a UIActivityViewController mail compose object used in a modal sharing action?

UIActivityViewController conforms to UIAppearance, so I thought the following does work but it doesn’t. The navigation bar stays default blue.
activityVC.navigationController.navigationBar.tintColor = [UIColor greyColor];
Context:
- (IBAction)share {
NSString *textToShare = ...;
NSURL *url = ...;
NSArray *activityItems = [[NSArray alloc] initWithObjects:textToShare,url,nil];
UIActivity *activity = [[UIActivity alloc] init];
NSArray *applicationActivities = [[NSArray alloc] initWithObjects:activity, nil];
UIActivityViewController *activityVC =
[[UIActivityViewController alloc] initWithActivityItems:activityItems applicationActivities:applicationActivities];
[activityVC setExcludedActivityTypes:[NSArray arrayWithObjects: UIActivityTypeMessage,UIActivityTypeCopyToPasteboard,
UIActivityTypeAssignToContact,UIActivityTypePostToWeibo, UIActivityTypePrint,UIActivityTypeSaveToCameraRoll,
nil]];
activityVC.navigationController.navigationBar.tintColor = [UIColor greyColor];
[self presentViewController:activityVC animated:YES completion:nil];
}
How can I change the tint of the UINavigationBar from blue to grey.
Do this in your Application Delegate it will change the color of navigation bar throughout the application where ever navigation bar is shown
you can also change the text color and font using this in your Application Delegate