it's not necessary to use a screensaver.sis, it is up to you to use a new screensaver. But if you are planning to use a new screensaver file, you should know its UID. Once I have tried to use a sreensaver file which i dont know the UID, then i had to delete my theme because it was newer compiled again. so try to make copies of your themes before doing something you dont know. As i said before you dont have write screensaver.sis and its UID.