--- a/src/pyams_skin/interfaces/__init__.py	Mon Oct 10 11:32:08 2016 +0200
+++ b/src/pyams_skin/interfaces/__init__.py	Mon Oct 10 15:27:10 2016 +0200
@@ -20,7 +20,7 @@
 
 # import packages
 from pyams_template.template import layout_config
-from zope.interface import implementer, Interface, Attribute
+from zope.interface import implementer, invariant, Interface, Attribute, Invalid
 from zope.configuration.fields import GlobalInterface
 from zope.schema import Text, TextLine, Choice, Int, Bool
 
@@ -62,9 +62,15 @@
 
     skin_parent = Attribute("Skin parent (local or inherited)")
 
-    skin = Choice(title=_("Presentation skin"),
-                  description=_("This skin will be used to handle presentation templates"),
-                  vocabulary='PyAMS user skins')
+    skin = Choice(title=_("Custom graphic theme"),
+                  description=_("This theme will be used to handle graphic design (colors and images)"),
+                  vocabulary='PyAMS user skins',
+                  required=False)
+
+    @invariant
+    def check_skin(self):
+        if not (self.skin or self.inherit_skin):
+            raise Invalid(_("You must select a custom skin or inherit from parent!"))
 
     def get_skin(self, request=None):
         """Get skin matching this content"""