Raise NotImplementedError for delete theme

Author: JonnyWong16

plexapi/mixins.py

@@ -597,10 +597,14 @@ def setTheme(self, theme):
         )
 
     def deleteTheme(self):
-        """ Delete the theme from a Plex object. """
-        key = f'/library/metadata/{self.ratingKey}/themes'
-        self._server.query(key, method=self._server._session.delete)
-        return self
+        """ Delete the theme from a Plex object.
+
+            Raises:
+                :exc:`~plexapi.exceptions.NotImplementedError`: Themes cannot be deleted through the API.
+        """
+        raise NotImplementedError(
+            'Themes cannot be deleted through the API.'
+        )
 
 
 class EditFieldMixin: