Update background image for all UINavigationBars

UpdateNavigationBarBackground.m

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    // Customize appearance
    [self customizeAppearance];
    ...
}
 
- (void)customizeAppearance
{
    // Set the background image for all UINavigationBars
    [[UINavigationBar appearance] setBackgroundImage:[UIImage imageNamed:@"navigation-bar"]
                                       forBarMetrics:UIBarMetricsDefault];
}