更改 UIAlertView 的背景颜色？

我想更改 UIAlertView 的背景颜色，但这似乎没有颜色属性。

AlertView的背景是一张图片 你可以改变这个图像

UIAlertView *theAlert = [[[UIAlertView alloc] initWithTitle:@"Atention"
   message: @"YOUR MESSAGE HERE", nil)
   delegate:nil cancelButtonTitle:@"OK" otherButtonTitles:nil] autorelease];

   [theAlert show];

   UILabel *theTitle = [theAlert valueForKey:@"_titleLabel"];
   [theTitle setTextColor:[UIColor redColor]];

   UILabel *theBody = [theAlert valueForKey:@"_bodyTextLabel"];
   [theBody setTextColor:[UIColor blueColor]];

   UIImage *theImage = [UIImage imageNamed:@"Background.png"];    
   theImage = [theImage stretchableImageWithLeftCapWidth:16 topCapHeight:16];
   CGSize theSize = [theAlert frame].size;

   UIGraphicsBeginImageContext(theSize);    
   [theImage drawInRect:CGRectMake(0, 0, theSize.width, theSize.height)];    
   theImage = UIGraphicsGetImageFromCurrentImageContext();    
   UIGraphicsEndImageContext();

   [[theAlert layer] setContents:[theImage CGImage]];