Customizar o background de todos os UINavigationBar

1. //Implemente este método no AppDelegate.m
2. //E chame o método no application:didFinishLaunchingWithOptions:
4. - (void)customizeAppearance
5. {
6.     UIImage * _navBarImage = [UIImage imageNamed:@"navBarBackground.png"];
7.     [[UINavigationBar appearance] setBackgroundImage:_navBarImage forBarMetrics:UIBarMetricsDefault];
8. }