Usually when you slide down your app screen, you will see gray, or white color background underneath your application.
You can change it by editing /platforms/ios/CoinWorth/Classes/MainViewController.m file.
Look for – (void)viewDidLoad and edit it like this:

- (void)viewDidLoad
{
    [super viewDidLoad];
    self.webView.backgroundColor = [UIColor colorWithRed:34/255.0 green:37/255.0 blue:43/255.0 alpha:1];
}

This will make the background dark-ish. 34, 37, 43 are rgb values, you need to divide them by 255.0 for UIColor.

To use black color( or any of the basic colors – just google UIColor ), simply do

- (void)viewDidLoad
{
    [super viewDidLoad];
    self.webView.backgroundColor = [UIColor blackColor];
}