How to change background color of storyboard in Xcode?

If you are referring to the white in your view controller:

• Select the View
• Go to inspector (right panel, 4th item)
• Change the background color field (section view)

I'm not sure you can do what you're trying to achieve. My best advice to you would be to create a dark coloured UIView which you can place behind the UILabel. Once you're happy you've got the UILabel setup the way you want it you can then just delete the UIView.