Install gtk-common-themes on Raspberry Pi Offline

I am trying to install "gtk-common-themes" for Snap offline on a Raspberry Pi 2B. It is imperative the device stays offline. On another Raspberry Pi 3 (online) machine, running the same OS (raspbian buster 10), I used snap download gtk-common-themes, copied the files to a USB drive, and put them on the Raspberry Pi 2. After running: sudo snap ack gtk-common-themes_1515.assert followed by sudo snap ack gtk-common-themes_1515.snap I get error: cannot assert: cannot decode request body into assertions: parsing assertion headers: header is not utf8. Can somebody please help me understand why this is happening? I have not had trouble with other snap packages.