Theft is theft. What theft is, fundamentally, is when a person enacts an action upon another in which violates the will of the other within a logical order of operations as it pertains to their property.
For example, if you find a diamond in the woods that nobody else even knows exists and you desire exclusive authority over it then you own it. You don't own it because I say so, you own it through a logical system because it literally cannot exist in any other feasible structure that isn't wholly arbitrary. You own it because you did not violate the will of another in order to own it, and you own it because you desire to own it.
If someone else came along and picked up a rock that was lying next to that diamond and you do not care at all about it, then you did not own that rock to begin with and thus, the other person did not rob you in order to claim ownership over that rock.
But if you are with that diamond and someone else comes along and thinks they deserve that diamond more than you but the only way they can manifest their desire of exclusive authority over the diamond is to engage in an action of which violates your will, they would need to steal it from you, and you cannot own that in which you've stolen, nor can you steal that in which you own.
No other claim can be made rationally that dictates who should own that diamond. It doesn't matter if the other person has 10 other diamonds and you have none. It doesn't matter if that other person has food and you do not and that diamond could get you food. None of that matters, and you actually agree that it doesn't matter because you already understand and act out in this world per a cardinal rule that you could never suffer another to violate your will, whatever it might be.
This is like the golden rule but 'The Golden Rule' is the ethical principle of treating others as you would want to be treated, but that concept is wholly flawed, because it does not take into account that humans have conflicting wills. Maybe I want to be punched in the fact, so the golden rule would then stipulate that I get to punch you in the face. Or what if you want a beautiful woman to have sex with you, do I get to rape her, then?
This principle is similar to the golden rule except that you have to understand that it is go unto others as THEY would want you to do to them, plus a little extra. More or less, human conflicts of wills are always contradictory and there are only two ways to resolve a will conflict, either arbitrary, or logically.
We both want X but only one of us can have X. How do we figure out who gets X?
Well I can murder you and take X, and that's arbitrary because I just decided I want X more which is not a quantifiable stance. OR we understand that if I had X first I would not want you to take it from me and since you have X first, realizing this, I choose not to take it from you. That is the fundamental principle of ownership vs. theft.
Theft is when will is violated as it pertains to ownership. Ownership is a desire to own where you did not violate the will of another to obtain exclusive authority over. it's that simple.
Everything else is nonsense.