Correct handling of default widget background color for label (#3104)

Explicitly use the default background color, rather than None as a reset value.

Author: proneon267

changes/3104.misc.rst

@@ -0,0 +1 @@
+The default background color of `toga.Label` on iOS now correctly displays a transparent background, instead of a black background.

iOS/src/toga_iOS/widgets/base.py

@@ -13,6 +13,10 @@ def __init__(self, interface):
         self.native = None
         self.create()
 
+        # Override this attribute to set a different default background
+        # color for a given widget.
+        self._default_background_color = self.native.backgroundColor
+
     @abstractmethod
     def create(self): ...
 
@@ -87,7 +91,9 @@ def set_color(self, color):
         pass
 
     def set_background_color(self, color):
-        self.native.backgroundColor = None if color is None else native_color(color)
+        self.native.backgroundColor = (
+            self._default_background_color if color is None else native_color(color)
+        )
 
     # INTERFACE
     def add_child(self, child):