MAGETWO-32027: Update theme.xml and theme.xsd

- updated them.xsd with definitions for parent
- updated theme.xml with parent reference

Author: Bibu Mathew

app/design/frontend/Magento/luma/theme.xml

@@ -5,6 +5,7 @@
 -->
 <theme xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="../../../../../lib/internal/Magento/Framework/Config/etc/theme.xsd">
     <title>Magento Luma</title>
+    <parent>Magento/blank</parent>
     <media>
         <preview_image>media/preview.jpg</preview_image>
     </media>

lib/internal/Magento/Framework/Config/etc/theme.xsd

@@ -15,6 +15,7 @@
         <xs:complexType>
             <xs:sequence>
                 <xs:element name="title" type="title"/>
+                <xs:element name="parent" type="parent_theme" minOccurs="0"/>
                 <xs:element name="media" type="media" minOccurs="0"/>
             </xs:sequence>
         </xs:complexType>
@@ -26,6 +27,12 @@
         </xs:restriction>
     </xs:simpleType>
 
+    <!-- Short theme name: '<theme>'. Fully-qualified theme name: '<path>/<theme>'. -->
+    <xs:simpleType name="parent_theme">
+        <xs:restriction base="xs:string">
+            <xs:pattern value="([^/]+/)?[^/]+"></xs:pattern>
+        </xs:restriction>
+    </xs:simpleType>
 
     <xs:complexType name="media">
         <xs:sequence>