Add display name for the "theme" plugin label (#4806)

* Add display name for the "theme" plugin label

* Update core/src/main/resources/hudson/model/Messages.properties

core/src/main/resources/hudson/model/Messages.properties

@@ -324,6 +324,7 @@ UpdateCenter.PluginCategory.scm-related=Source Code Management related
 UpdateCenter.PluginCategory.security=Security
 UpdateCenter.PluginCategory.slaves=Agent Launchers and Controllers
 UpdateCenter.PluginCategory.test=Testing
+UpdateCenter.PluginCategory.theme=UI Themes
 UpdateCenter.PluginCategory.trigger=Build Triggers
 UpdateCenter.PluginCategory.ui=User Interface
 UpdateCenter.PluginCategory.upload=Artifact Uploaders