What is called a steam "style" computer case means that it only has the appearance of a steam locomotive; it's not actually powered by steam.
Mr. Hildebrandt is the owner and modder of this Victorian-style retrofitted case (named Telecalculograph). Unfortunately, Mr. Hildebrandt hasn't provided detailed descriptions of his creation, so we can only roughly guess based on the pictures that this case has turned all the original buttons on the shell into toggle switches, replaced all digital information display screens with analog dials, and the large circular disc on the front of the case (the one with the four white bars) can convincingly pretend to be the furnace of a steam engine.
We hope that Mr. Hildebrandt will continue to add some characteristic functions that a steam locomotive should have, making it more like a solid and heavy-duty piece from the Victorian era. For example, something like this: when he occasionally falls into the kind of boredom akin to Mr. Thenew, he could pull a handle to let the Telecalculograph emit a long whistle and spout thick smoke from the chimney for his own entertainment.