No, it's not. There are two reasons why.

First, theft implies that the item and use of the item is lost to the original owner. I am simply making a copy in this case. They get to keep the original item.

Now, some may claim that by not purchasing, I am making them lose revenue. That is only true if I am downloading something that I would have purchased, which has never been the case for me. In addition, this argument is easily refuted by the second position below.

Second, and more importantly, when I download something, I am taking something that somebody has offered to me. If I purchase a DVD, for example, I can easily give it to a friend as a gift, and it now belongs to him. Filesharing is similar to this. If I download something, I am taking something that somebody who either purchased the document or obtained the document legally from someone else. That person is giving me access to something that he has. Even if this is lost revenue, it doesn't matter because so is giving the DVD to my friend, which is legal.

Such things are not property, but patterns of property.

If you can "steal a pattern", then someone else can control your property.