What's being stolen in theft of electricity? The electrons or the holes?

Service. This question "is a copy equivalent and trivial to create?" is a pretty good way to understand the line in the sand regarding what stuff is subject to copyright infringement versus theft. We have theft of property and theft of service. We do not have theft of trivially copyable stuff because we have copyright infringement instead for that case.

Paintings, manuscripts, etc. are subject to theft because they are nonfungible. The content itself is subject to copyright because the copies are fungible.