如何在ios 8.3中设置MFMailComposeViewController导航栏backgroundImage

Question

在AppDelegate didFinishLaunchingWithOptions，

[[UIApplication sharedApplication] setStatusBarStyle:UIStatusBarStyleLightContent];
[[UINavigationBar appearance] setBackgroundImage:[UIImage imageNamed:@"Topbar"] forBarMetrics:UIBarMetricsDefault];

但是当我回复系统电子邮件时：

 MFMailComposeViewController *mc = [[MFMailComposeViewController alloc] init];
 [mc.navigationBar setTintColor:[UIColor whiteColor]];
 [mc.navigationController.navigationBar setBackgroundImage:[UIImage imageNamed:@"Topbar"] forBarMetrics:UIBarMetricsDefault];

[self presentViewController:mc animated:YES completion:^{
         [[UIApplication sharedApplication] setStatusBarStyle:UIStatusBarStyleLightContent animated:NO];}];

但MFMailComposeViewController也是白色