Added ability to disable theming globally

Sources/iOS/Theme.swift

@@ -74,6 +74,9 @@ public struct Theme: Hashable {
   /// Text and iconography color to be used on error color.
   public var onError = Color.white
   
+  /// A boolean indicating if theming is enabled globally.
+  public static var isEnabled = true
+  
   /// An initializer.
   public init() { }
 }
@@ -175,7 +178,7 @@ public extension Themeable where Self: NSObject {
   /// A class-wide boolean indicating if theming is enabled.
   static var isThemingEnabled: Bool {
     get {
-      return AssociatedObject.get(base: self, key: &IsThemingEnabledKey) {
+      return Theme.isEnabled && AssociatedObject.get(base: self, key: &IsThemingEnabledKey) {
         true
       }
     }