how to locate the name for the icons of the snap packages
ARe you author of Sierra CT?
There is no way to apply icons to snap-built apps, because they are absolute icons and hard-coded to png. The users who use your icons theme have to copy the snap-built apps desktop file to
~/.local/share/applications and modify the icons path.
You can read what I explained in my Suru++ 20 description about snap, Flatpak and AppImage apps icons:
Read the answer of @linusg,
The desktop files of snap apps are in the “HDD” snapd.