Pop is intended to be installed through the package manager. Packages for Pop are available in PPA:
sudo add-apt-repository ppa:system76/pop
sudo apt update
sudo apt install pop-theme
It's recommended to use the pop-theme metapackage, as this will pull in all components of the look. However, individual components can be installed separately, e.g:
sudo apt install pop-gtk-theme
It's also recommended to restart the GNOME Shell after applying the theme of your choice.
Enter the Shell's command launcher
Alt + F2
This will restart the Shell after you hit Enter
r
sudo apt install ppa-purge
bound the repo again in.
to remove use
sudo ppa-purge ppa:system76/pop
This will disable the ppa and set package version back to ubuntu one's
Sources:
https://github.com/pop-os/gtk-theme/blob/master/README.md
https://askubuntu.com/questions/1184253/installing-and-removing-the-pop-theme-leaves-package-versions-referring-to-the-s