Correct handling of color for non-button widgets.

gtk/tests_backend/widgets/button.py

@@ -1,3 +1,4 @@
+from toga.colors import TRANSPARENT
 from toga_gtk.libs import Gtk
 
 from .base import SimpleProbe
@@ -13,8 +14,8 @@ def text(self):
     @property
     def background_color(self):
         color = super().background_color
-        # Background color of
-        if color.r == 0 and color.g == 0 and color.b == 0 and color.a == 0.0:
+        # Background color of TRANSPARENT is treated as a reset.
+        if color == TRANSPARENT:
             return None
         return color
 

gtk/tests_backend/widgets/properties.py

@@ -1,18 +1,24 @@
 from travertino.fonts import Font
 
-from toga.colors import rgba
+from toga.colors import TRANSPARENT, rgba
 from toga.style.pack import CENTER, JUSTIFY, LEFT, RIGHT
 from toga_gtk.libs import Gtk, Pango
 
 
 def toga_color(color):
     if color:
-        return rgba(
+        c = rgba(
             int(color.red * 255),
             int(color.green * 255),
             int(color.blue * 255),
             color.alpha,
         )
+
+        # Background color of rgba(0,0,0,0.0) is TRANSPARENT.
+        if c.r == 0 and c.g == 0 and c.b == 0 and c.a == 0.0:
+            return TRANSPARENT
+        else:
+            return c
     else:
         return None