title: how to customize QLPreviewController's navBar and toolbar tintColor
body: <pre><code>QLPreviewController * preview = [[QLPreviewController alloc] init];
  preview.dataSource = self;
  preview.currentPreviewItemIndex = sender.tag;
  preview.editing= YES; 
  [self presentModalViewController:preview animated:YES];
  [preview release];
  </code></pre>
  
  <p>These two lines does not work for me. so be careful before writing these lines.</p>
  
  <pre><code>[preview.tabBarController.tabBar setTintColor:[UIColor blackColor]];
  [preview navigationController].navigationBar setTintColor: [UIColor blackColor]];
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/xFro2.png" alt="Problem Screenshot here"></p>