The definition of theft is the aggressive seizure of property*. Wealth is the sum of economic goods in one’s possession; however, in the context of inflation, in which this discussion is framed, wealth is implicitly defined as the value of the sum of one’s money. Since the value of a good is not objective, it cannot be theft to inflate the money supply as no property is seized; theft doesn’t apply to the situation.
Ultimately, if you wish for theft to apply to the situation at hand, inflation, you must contend that one has property rights to the value of his goods, in this case the property rights to the value of his money, which I have already demonstrated is argumentatively unjustifiable. In the context of inflation, ‘increasing one’s wealth at another’s expense’ translates to “increasing the quantity of goods that one has, through the use of his own means (ink, paper, printing press, et cetera in this case) with the consequence, intended or not, of diminishing the value of the units of these goods held by others, ceteris paribus.” This elaborated translation is different from one in which I gain at your expense by, say, involuntarily seizing your computer which would be an example of theft.
*I explain my omission of the word ‘physical’ from this definition below.
I use the word ‘physical’ in an attempt to emphasize that property has intersubjectively ascertainable, or objective, boundaries (I should have been clearer here and said as much; my mistake, thanks for catching it). Bitcoins are recognizable units of a means of exchange; if someone is capable of claiming that whatever quantity of bitcoins are his, and he has acquired them legitimately (i.e. through homesteading, production or voluntary exchange), then they are his property since they are intersubjectively ascertainable (i.e. Bill has 1 bitcoin) because a unit can be distinguished from another (this is so by the fact that you are capable of taking another’s bitcoins, which means that you can recognize that they are part of his property). Of course, if you involuntarily take these from someone, it is theft.