So I discovered this awesome theme. Installation was quite smooth and it looked gorgeous.
But after sudo apt update, I found hundreds of packages to be upgraded. Weird. It turned out that the PPA contains not only the theme but also other packages for Pop!_OS. To prevent installing/upgrading other packages accidentally, I gave the PPA a lower priority:
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sudo tee <<EOF /etc/apt/preferences.d/system76-pop-pin >/dev/null
Package: *
Pin: release o=LP-PPA-system76-pop
Pin-Priority: 400
Package: pop-theme
Pin: release o=LP-PPA-system76-pop
Pin-Priority: 600
Package: pop-gnome-shell-theme
Pin: release o=LP-PPA-system76-pop
Pin-Priority: 600
Package: pop-gtk-theme
Pin: release o=LP-PPA-system76-pop
Pin-Priority: 600
Package: pop-icon-theme
Pin: release o=LP-PPA-system76-pop
Pin-Priority: 600
Package: pop-sound-theme
Pin: release o=LP-PPA-system76-pop
Pin-Priority: 600
EOF