Perhaps that is what it is, but this is a very old garment, judging by the mothing, colour variation and fading, and it is of very high quality and detail. A costume or theatrical garment is usually not made with such attention to detail and care. I'd rather spend some time on research. The photo isn't a very good one for making a pronouncement upon.
My initial thought was was to see if I could find the larger buttons (~22mm) to replace the ones on the front and rear, with buttons that match the ones on the cuffs, but the buttons appear to cost ~$40 to $50 each, and I didn't want to invest that much in them without knowing whether they would be correct for the jacket. As mentioned in my previous post, the cuff buttons are U.S. Army General Staff Officer buttons, which were designed in 1836, and worn through the time of the Civil War. Although 4 of the 6 cuff buttons could be original to the jacket, 2 are replacements, but authentic.
I would have guessed that a costume or theatrical garment would have used flat, inexpensive brass buttons.