I downloaded the Pupi theme, unpacked the zip, dropped the directory into /usr/share/themes, and rebooted. The new theme still does not show up in the list of 'Styles' in the settings. If someone has some other suggesting to adapt Mint/Xfce to a Retina display at readable sizes that would also be helpful.
This is a window manager theme (Xfce window manager), as you can tell by the presence of only the xfwm4 folder inside the theme zip.
You can access it in Settings Manager -> Window Manager -> Style
As an alternative install location, if you are the only user in your system, create a .themes (mind the dot) folder inside your home directory and unzip your theme there.
(Tested on Xfce 4.10 & 4.12)