There is a lovely GTK3 theme available for the Linux Mint MATE desktop. This looks very nice when coupled with a good Metacity theme.
This is what the El Capitan theme looks like on a Debian desktop.
Download the El Capitan theme here. https://dl.opendesktop.org/api/files/download/id/1474234465/OS.X.El.Capitan.v0.9.tar.gz.
This is a very nice looking theme and well worth adding to your desktop.
Mac OSX Sierra theme for Linux Mint MATE. Another nice option to theme your Linux desktop.
https://dl.opendesktop.org/api/files/download/id/1484231496/macOS%20Sierra%20v2.2.zip.
Mac OSX dark theme.
https://dl.opendesktop.org/api/files/download/id/1484231723/macOS%20Dark%20v2.2.zip.
Fedora Core Bluecurve theme for Linux Mint MATE.
https://dl.dropboxusercontent.com/u/105936874/Bluecurve-Phenix.tar.gz.
This theme works perfectly with the MATE desktop. This allows you to have a classic Fedora Core styled desktop with Linux Mint MATE or Debian. This is well worth installing.
I loved using the old Fedora Core desktops with the Bluecurve theme and it is great to install this theme on a modern Linux desktop.