Adds support for Dark Mode on iOS 13 (#170)

Fixes #168 - iOS 13 Dark Mode

Example/Swift/Bulletin/BulletinDataSource.swift

@@ -225,8 +225,16 @@ enum BulletinDataSource {
         let page = BLTNPageItem(title: "Setup Completed")
         page.image = #imageLiteral(resourceName: "IntroCompletion")
         page.imageAccessibilityLabel = "Checkmark"
-        page.appearance.actionButtonColor = #colorLiteral(red: 0.2980392157, green: 0.8509803922, blue: 0.3921568627, alpha: 1)
-        page.appearance.imageViewTintColor = #colorLiteral(red: 0.2980392157, green: 0.8509803922, blue: 0.3921568627, alpha: 1)
+
+        let tintColor: UIColor
+        if #available(iOS 13.0, *) {
+            tintColor = .systemGreen
+        } else {
+            tintColor = #colorLiteral(red: 0.2980392157, green: 0.8509803922, blue: 0.3921568627, alpha: 1)
+        }
+        page.appearance.actionButtonColor = tintColor
+        page.appearance.imageViewTintColor = tintColor
+
         page.appearance.actionButtonTitleColor = .white
 
         page.descriptionText = "PetBoard is ready for you to use. Happy browsing!"

Sources/Appearance/BLTNItemAppearance.swift

@@ -13,8 +13,14 @@ import UIKit
 
     // MARK: - Color Customization
 
-    /// The tint color to apply to the action button (default blue).
-    @objc public var actionButtonColor: UIColor = #colorLiteral(red: 0, green: 0.4784313725, blue: 1, alpha: 1)
+    /// The tint color to apply to the action button (default `.link` on iOS 13 and `.blue` on older systems).
+    @objc public var actionButtonColor: UIColor = {
+        if #available(iOS 13.0, *) {
+            return .link
+        } else {
+           return #colorLiteral(red: 0, green: 0.4784313725, blue: 1, alpha: 1)
+        }
+    }()
 
     /// The button image to apply to the action button
     @objc public var actionButtonImage: UIImage?
@@ -28,23 +34,47 @@ import UIKit
     /// The border width to apply to action button.
     @objc public var actionButtonBorderWidth: CGFloat = 1.0
 
-    /// The title color to apply to the alternative button (default blue).
-    @objc public var alternativeButtonTitleColor: UIColor = #colorLiteral(red: 0, green: 0.4784313725, blue: 1, alpha: 1)
+    /// The title color to apply to the alternative button (default `.link` on iOS 13 and `.blue` on older systems).
+    @objc public var alternativeButtonTitleColor: UIColor = {
+        if #available(iOS 13.0, *) {
+            return .link
+        } else {
+            return #colorLiteral(red: 0, green: 0.4784313725, blue: 1, alpha: 1)
+        }
+    }()
 
     /// The border color to apply to the alternative button.
     @objc public var alternativeButtonBorderColor: UIColor? = nil
 
     /// The border width to apply to the alternative button.
     @objc public var alternativeButtonBorderWidth: CGFloat = 1.0
 
-    /// The tint color to apply to the imageView (if image rendered in template mode, default blue).
-    @objc public var imageViewTintColor = #colorLiteral(red: 0, green: 0.4784313725, blue: 1, alpha: 1)
+    /// The tint color to apply to the imageView (if image rendered in template mode, default `.link` on iOS 13 and `.blue` on older systems).
+    @objc public var imageViewTintColor: UIColor = {
+        if #available(iOS 13.0, *) {
+            return .link
+        } else {
+            return #colorLiteral(red: 0, green: 0.4784313725, blue: 1, alpha: 1)
+        }
+    }()
 
-    /// The color of title text labels (default light gray).
-    @objc public var titleTextColor = #colorLiteral(red: 0.568627451, green: 0.5647058824, blue: 0.5725490196, alpha: 1)
+    /// The color of title text labels (default `.secondaryLabel` on iOS 13 and light gray on older systems).
+    @objc public var titleTextColor: UIColor = {
+        if #available(iOS 13.0, *) {
+            return .secondaryLabel
+        } else {
+            return #colorLiteral(red: 0.568627451, green: 0.5647058824, blue: 0.5725490196, alpha: 1)
+        }
+    }()
 
-    /// The color of description text labels (default black).
-    @objc public var descriptionTextColor = #colorLiteral(red: 0, green: 0, blue: 0, alpha: 1)
+    /// The color of description text labels (default `.label` on iOS 13 and black on older systems).
+    @objc public var descriptionTextColor: UIColor = {
+        if #available(iOS 13.0, *) {
+            return .label
+        } else {
+            return #colorLiteral(red: 0, green: 0, blue: 0, alpha: 1)
+        }
+    }()
 
     // MARK: - Corner Radius Customization
 

Sources/BLTNItemManager.swift

@@ -27,12 +27,19 @@ import UIKit
     // MARK: - Background
 
     /**
-     * The background color of the bulletin card. Defaults to white.
+     * The background color of the bulletin card. Defaults to `systemBackground` on iOS 13
+     * and white on older versions of the OS.
      *
      * Set this value before presenting the bulletin. Changing it after will have no effect.
      */
 
-    @objc public var backgroundColor: UIColor = #colorLiteral(red: 1.0, green: 1.0, blue: 1.0, alpha: 1.0)
+    @objc public var backgroundColor: UIColor = {
+        if #available(iOS 13.0, *) {
+            return .systemBackground
+        } else {
+            return .white
+        }
+    }()
 
     /**
      * The style of the view covering the content. Defaults to `.dimmed`.
@@ -247,21 +254,30 @@ extension BLTNItemManager {
      * Displaying the loading indicator does not change the height of the page or the current item. It will disable
      * dismissal by tapping and swiping to allow the task to complete and avoid resource deallocation.
      *
-     * - parameter color: The color of the activity indicator to display. Defaults to black.
+     * - parameter color: The color of the activity indicator to display. Defaults to .label on iOS 13 and .black on older systems.
      *
      * Displaying the loading indicator does not change the height of the page or the current item.
      */
 
-    @objc public func displayActivityIndicator(color: UIColor = #colorLiteral(red: 0, green: 0, blue: 0, alpha: 1)) {
+    @objc public func displayActivityIndicator(color: UIColor? = nil) {
 
         assertIsPrepared()
         assertIsMainThread()
 
         shouldDisplayActivityIndicator = true
-        lastActivityIndicatorColor = color
+        lastActivityIndicatorColor = color ?? defaultActivityIndicatorColor
 
-        bulletinController.displayActivityIndicator(color: color)
+        bulletinController.displayActivityIndicator(color: lastActivityIndicatorColor)
+    }
 
+    /// Provides a default color for activity indicator views.
+    /// Defaults to .label on iOS 13 and .black on older systems.
+    private var defaultActivityIndicatorColor: UIColor {
+        if #available(iOS 13.0, *) {
+            return .label
+        } else {
+            return .black
+        }
     }
 
     /**