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UIAlertViewの背景色をカスタマイズする必要があります。iphone UIAlertView - カスタム背景色
iphone 3.xで完全に動作するサンプルコードがありますが、iPhone 4ではalertviewにデフォルトの青色が表示されます。
UIAlertView *alertView =
[[[UIAlertView alloc] initWithTitle:@"Alerta"
message:msg
delegate:nil
cancelButtonTitle:@"OK"
otherButtonTitles:nil] autorelease];
[alertView show];
UILabel *theTitle = [alertView valueForKey:@"_titleLabel"];
[theTitle setTextColor:[UIColor redColor]];
UILabel *theBody = [alertView valueForKey:@"_bodyTextLabel"];
[theBody setTextColor:[UIColor whiteColor]];
UIImage *theImage = [UIImage imageNamed:@"Background.png"];
theImage = [theImage stretchableImageWithLeftCapWidth:0 topCapHeight:0];
CGSize theSize = [alertView frame].size;
UIGraphicsBeginImageContext(theSize);
[theImage drawInRect:CGRectMake(0, 0, theSize.width, theSize.height)];
theImage = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
[[alertView layer] setContents:(id)theImage.CGImage];
[alertView show];