User permissions

I am wondering if there is a way to have users with different roles/permissions. I have set up an Admin user who can contribute items and make them public. This user is just uploading jpg files. Once the images are loaded, we would like the student research to edit the images and create the metadata for them. If I create the users with the contributor role, I don't want them to be able to delete the items. If I use the contributions with guest user on the public page, it doesn't seem like they can edit records that already exist. So, I am confused as to how best I can get users to be able to edit records that already exist but not be able to delete them or add new items of their own.

The roles/permissions mostly hinge around ownership of an item, so that the administrator role is the one that can edit items created by other people. But that also gives the permission to delete and create items. Out of the box, that's a granular as it gets.

Guest user is the most minimal user role -- it only allows for an account to exist, but it has no permissions to do anything.

What you are talking about could probably be created with a plugin that creates a new role and assigns permissions.

I am new to Omeka, so I am still trying to figure out what it can do and I am trying to use it to accomplish the things I need without having to do a lot of new coding, I prefer to modify existing code if possible.

Is it possible to use different admin themes for example copy the admin default theme to admin student theme where that theme did not have the button for delete? Is there a table with the users that state which theme they are using, so that you could say for user A they have theme admin owner theme and user B has admin creator theme?

There's not really a way to switch themes based on the user or role, nor a way to have different admin themes available.