A UIView extension that adds a style property which allows you to style UI elements using tags.

Stylesheet.swift

// Default button style
StyleCollection.addStyle("btn-default", paint: {(view) -> Void in
    
    // These buttons should all have rounded corners and a dropshadow
    view.layer.cornerRadius = view.frame.height * 0.5
    view.layer.backgroundColor = UIColor(hex:0x294A5F).CGColor
    
    view.layer.shadowColor = UIColor.blackColor().CGColor
    view.layer.shadowOffset = CGSizeMake(0, 2)
    view.layer.shadowRadius = 2
    view.layer.shadowOpacity = 0.15
    
    if let button = view as? UIButton {
        button.setTitleColor(UIColor.whiteColor(), forState: .Normal)
    }
})

// Attention button style
StyleCollection.addStyle("btn-attention", paint: {(view) -> Void in

    // These buttons should feature a more -in your face- background color
    view.layer.backgroundColor = UIColor(hex:0xEE638B).CGColor
})

UIViewStyle.swift

// Collection of defined styles
private var styles:Dictionary<String,((view:UIView) -> Void)> = Dictionary()

// StyleCollection class
class StyleCollection {
    
    class func addStyle(style:String, paint:((view:UIView) -> Void)) -> Void {
        styles[style] = paint
    }
    
    class func applyStyle(style:String?, view:UIView) -> Void {
        
        if (style == nil) {
            return
        }
        
        let styleList = style!.characters.split{$0 == " "}.map(String.init)
        for style:String in styleList {
            applyStyle(style, view:view)
        }

    }
    
    private class func applyStyle(style:String, view:UIView) -> Void {
        if let paint = styles[style] {
            paint(view:view)
        }
    }
    
}

// Adds style property to UIView so styles can be applied
extension UIView {
    
    @IBInspectable var style: String! {
        get {
            return nil
        }
        set(newValue) {
            StyleCollection.applyStyle(newValue, view:self)
        }
    }
    
}

You can link styles to UI elements with the style property (in code or on storyboard).

var buttonBuyProduct = UIButton()
buttonBuyProduct.style = "btn-default btn-attention"

As a front-end developer used to styling UI elements with CSS I was really missing something similar in Swift.