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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.