Ok. First, get the deb package for version 18.04. Go to the PopOS PPA, click on “view package details” and find the “pop-gtk-theme” in the list (it should be on page 2). The one you want is the line with “Bionic”, so "pop-gtk-theme - 3.1.2~1535402967~18.04~cc107bf ". Click on the arrow/triangle on the left to show the details. Scroll down to “Package files” and click on the link to the deb package (pop-gtk-theme_3.1.2~1535402967~18.04~cc107bf_all.deb) to download it.
Open a Caja (file manager) window and go to the dir where it’s been downloaded (should be ~/Download), right click on the deb package and chose “extract here” to uncompress it with Engrampa (the archive manager). Go inside the new dir you get (pop-gtk-theme_3.1.2~1535402967~18.04~cc107bf_all) → usr → share → themes and you’ll see six directories that match the theme variants. I’ve noticed that between v. 18.04 and 18.10, they renamed two of the themes (Pop-dark-slim and Pop-light-slim became Pop-slim-dark and Pop-slim-light), so you’ll have to adapt.
Open another Caja window and go to ~/.local/share/themes (if you don’t have a themes directory in ~/.local/share, just create it). Inside, create these six dirs: Pop, Pop-dark, Pop-light, Pop-slim, Pop-slim-dark and Pop-slim-light. Then, copy the metacity-1 directory from each of the themes extracted (Caja windows #1) to the corresponding dir in ~/.local/share/themes (Caja window #2). As they renamed two themes dirs, the metacity-1 from Pop-dark-slim and Pop-light-slim will go in Pop-slim-dark and Pop-slim-light instead, of course.
When it’s done, check in the Appearance settings if you still have the error. If not, then you’re good and you can stop here.
If you still have the errors, that means that unlike what I thought, the themes engine can’t mix themes parts from two different places, in which case you’ll have to copy them where the themes are actually installed in the system:
sudo cp -R ~/.local/share/themes/Pop/metacity-1 /usr/share/themes/Pop/
sudo cp -R ~/.local/share/themes/Pop-dark/metacity-1 /usr/share/themes/Pop-dark/
sudo cp -R ~/.local/share/themes/Pop-light/metacity-1 /usr/share/themes/Pop-light/
sudo cp -R ~/.local/share/themes/Pop-slim/metacity-1 /usr/share/themes/Pop-slim/
sudo cp -R ~/.local/share/themes/Pop-slim-dark/metacity-1 /usr/share/themes/Pop-slim-dark/
sudo cp -R ~/.local/share/themes/Pop-slim-light/metacity-1 /usr/share/themes/Pop-slim-light/
When it’s done, you can delete the six dirs in ~/.local/share/themes. Then, it should work.
Putting the missing themes parts in ~/.local/share/themes (if it works) has the upside of not fiddling with the files managed by the package at system level. The downside is that if you have several users on your PC, you have to copy them for each user if they want to use the Pop themes.
Putting them at system level has the upside of having them ready for every user but the downside of having “foreign” files in a place managed by a deb package, so if you uninstall it, it’ll keep the files you’ve put manually and you’ll have to delete them yourself (and the dir for each theme that won’t have been removed because it wasn’t empty). Also, if at some point they add the metacity-1 subdir back in each theme, you’ll have errors when updating the package and you’ll have to clean the files yourself (delete the ones you’ve added manually) before forcing the reinstallation of the package.
So, it’s highly recommended to use the first option if it works, it’s far easier to manage.
