Lightweight styling setup using UIAppearance

Example.swift

import PlaygroundSupport

// using constrained static members on `Style` we can create presets
extension Style where T: UIButton {
    static var global: Style<UIButton> {
        return .init { button in
            button.backgroundColor = .blue
            button.setTitleColor(.red, for: .normal)
        }
    }
    static var local: Style<UIButton> {
        return .init { button in
            button.backgroundColor = .green
        }
    }
}

// apply style to _all_ instances of UIButton
UIButton.styleAll(with: .global)

let view = UIView(frame: .init(x: 0, y: 0, width: 300, height: 500))
view.backgroundColor = .red

// apply style _only_ to this instance - notice that the styles have composed!
let button = UIButton(frame: .init(x: 25, y: 25, width: 250, height: 50)).styled(with: .local)

button.setTitle("Test", for: .normal)
view.addSubview(button)
PlaygroundPage.current.liveView = view

Styleable.swift

protocol Styleable: AnyObject { }

extension UIView: Styleable { }

extension Styleable where Self: UIView {
    func style(_ closure: (Self) -> Void) -> Self {
        closure(self)
        return self
    }
    func styled(with style: Style<Self>) -> Self {
        return self.style(style.applyStyle)
    }
    static func styleAll(_ closure: (Self) -> Void) {
        closure(appearance())
    }
    static func styleAll(with style: Style<Self>) {
        style.applyStyle(appearance())
    }
}

struct Style<T: UIView> {
    let applyStyle: (T) -> Void
}