Example of creating an App Theme using structs

theme.swift

public protocol ThemeScheme{
    var fonts: FontScheme {get}
    var colours: ColourScheme {get}
}

public struct Theme: ThemeScheme{
    
    public let fonts: FontScheme
    public let colours: ColourScheme
    
    public init(fonts: FontScheme = Fonts(), colours: ColourScheme = Colours()){
        self.fonts = fonts
        self.colours = colours
    }
}

public protocol ColourScheme{
    var primaryColor: UIColor {get} // default colour in scheme
    var textOnPrimaryColour: UIColor {get}
    
    var secondaryColor: UIColor {get} // ie. selected colour of controls
    
    var black: UIColor {get}
    var white: UIColor {get}
    
    var lightGrey: UIColor {get}
    var darkGrey: UIColor {get}
}

public struct Colours: ColourScheme{
    public let primaryColor = UIColor(red:0.17, green:0.49, blue:0.85, alpha:1.00)
    public let textOnPrimaryColour = UIColor(red:0.85, green:0.92, blue:0.89, alpha:1.00)
    
    public let secondaryColor = UIColor(red:0.99, green:0.19, blue:0.35, alpha:1.00)
    
    public let black = UIColor(red:0.08, green:0.06, blue:0.13, alpha:1.00)
    public let white = UIColor(red:0.97, green:0.97, blue:0.97, alpha:1.00)
    
    public let lightGrey = UIColor(red:0.84, green:0.84, blue:0.84, alpha:1.00)
    public let darkGrey = UIColor(red:0.56, green:0.56, blue:0.58, alpha:1.00)
}

public protocol FontScheme{
    var navigationTitle: UIFont {get}
    
    var cellHeaderText: UIFont {get}
    var cellImageCaptionText: UIFont {get}
    var cellImageSmallCaptionText: UIFont {get}
    
    var headerText: UIFont {get}
    var bodyText: UIFont {get}
    var captionText: UIFont {get}
    
    var textField: UIFont {get}
    
    var searchBar: UIFont {get}
    
    var button: UIFont {get}
}

public struct Fonts: FontScheme {
    public let navigationTitle = UIFont(name: "AvenirNextCondensed-DemiBold", size: 25)!
    
    public let cellHeaderText = UIFont(name: "Avenir-Book", size: 29)!
    public let cellImageCaptionText = UIFont(name: "Avenir-Light", size: 17)!
    public let cellImageSmallCaptionText = UIFont(name: "Avenir-Heavy", size: 17)!
    
    public let headerText = UIFont(name: "AvenirNext-Medium", size: 15)!
    public let bodyText = UIFont(name: "Avenir-Light", size: 15)!
    public let captionText = UIFont(name: "Avenir-Light", size: 15)!
    public let hyperlinkText = UIFont(name: "Avenir-Medium", size: 15)!
    
    public let textField = UIFont(name: "Avenir-Book", size: 27)!
    
    public let searchBar = UIFont(name: "Avenir-Light", size: 17)!
    
    public let button = UIFont(name: "AvenirNext-Regular", size: 17)!
}