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.
Mac OSX dark theme.
Fedora Core Bluecurve theme for Linux Mint MATE.
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.