Added upgrade step to reload the theming registry.

Otherwise when you upgrade from Plone 5.2.1, and try to save the theming control panel, you get an error:

```
Traceback (innermost last):
  Module ZPublisher.WSGIPublisher, line 162, in transaction_pubevents
  Module ZPublisher.WSGIPublisher, line 359, in publish_module
  Module ZPublisher.WSGIPublisher, line 262, in publish
  Module ZPublisher.mapply, line 85, in mapply
  Module ZPublisher.WSGIPublisher, line 63, in call_object
  Module plone.app.theming.browser.controlpanel, line 65, in __call__
  Module plone.app.theming.browser.controlpanel, line 215, in update
  Module plone.registry.recordsproxy, line 59, in __setattr__
AttributeError: custom_css_timestamp
```

src/plone/app/theming/configure.zcml

@@ -26,15 +26,6 @@
         provides="Products.GenericSetup.interfaces.EXTENSION"
         />
 
-    <gs:upgradeStep
-        title="Update registry"
-        source="*"
-        destination="1000"
-        handler=".upgrade.update_registry"
-        sortkey="1"
-        profile="plone.app.theming:default"
-        />
-
     <gs:upgradeStep
         title="Combine Theming control panels"
         source="1000"
@@ -44,6 +35,14 @@
         profile="plone.app.theming:default"
         />
 
+    <gs:upgradeStep
+        title="Update registry"
+        source="1001"
+        destination="1002"
+        handler=".upgrade.update_registry"
+        profile="plone.app.theming:default"
+        />
+
     <!-- Transform order 8850 - apply theme transform -->
     <adapter
         name="plone.app.theming.transform"

src/plone/app/theming/profiles/default/metadata.xml

@@ -1,5 +1,5 @@
 <metadata>
-    <version>1001</version>
+    <version>1002</version>
     <dependencies>
         <dependency>profile-plone.app.registry:default</dependency>
     </dependencies>