Lighter button fill in light mode (#489)

Provides a lighter fill color for the button when in light mode and
handles updating it dynamically as the appearance changes.

BeeKit/UI/BSButton.swift

@@ -13,23 +13,62 @@ public class BSButton : UIButton {
 
     required init?(coder aDecoder: NSCoder) {
         super.init(coder: aDecoder)
+
+        registerForTraitChanges(
+            [UITraitUserInterfaceStyle.self]) {
+                (self: Self, previousTraitCollection: UITraitCollection) in
+                self.resetStyle()
+            }
+
         self.setUp()
     }
 
     override init(frame: CGRect) {
         super.init(frame: frame)
+
+        registerForTraitChanges(
+            [UITraitUserInterfaceStyle.self]) {
+                (self: Self, previousTraitCollection: UITraitCollection) in
+                self.resetStyle()
+            }
+
         self.setUp()
     }
 
     private func setUp() {
         self.titleLabel?.font = UIFont.beeminder.defaultBoldFont
-        self.setTitleColor(UIColor.Beeminder.yellow, for: UIControl.State())
-        self.tintColor = .black
+        self.setTitleColor(dynamicTitleColor, for: UIControl.State())
+        self.tintColor = dynamicTintFillColor
         self.configuration = .filled()
 
         self.layer.borderColor = UIColor.Beeminder.yellow.cgColor
         self.layer.borderWidth = 1
         self.layer.cornerRadius = 4
     }
 
+    private func resetStyle() {
+        self.tintColor = dynamicTintFillColor
+        self.setTitleColor(dynamicTitleColor, for: UIControl.State())
+    }
+
+    private let dynamicTintFillColor = UIColor { traitCollection in
+        switch traitCollection.userInterfaceStyle {
+        case .dark:
+            return UIColor.black
+        default:
+            return UIColor(red: 235.0/255.0,
+                           green: 235.0/255.0,
+                           blue: 235.0/255.0,
+                           alpha: 1.0)
+        }
+    }
+
+    private let dynamicTitleColor = UIColor { traitCollection in
+        switch traitCollection.userInterfaceStyle {
+        case .dark:
+            return UIColor.Beeminder.yellow
+        default:
+            return UIColor.black
+        }
+    }
 }