There are countless themes for the Linux MATE desktop. Here is a selection of the nicest themes.
Prof Gnome Theme.
https://www.mate-look.org/p/1334194
This is a very nice professional GTK theme that will work with the MATE desktop.
Material Black Colors desktop.
https://www.mate-look.org/p/1316887
This theme is a new GTK, xfwm4, GNOME-Shell, and Cinnamon- Dark Mode Theme. This has a matching icon theme and a theme for the folders. This would be a great theme to make the desktop look amazing.
Qogir theme for GTK.
https://www.mate-look.org/p/1230631
A flat design theme for MATE. This would provide a very nice flat look for the whole MATE desktop.
This theme would work on Gnome Shell, MATE, Cinnamon and other GTK-based desktops.
Redmond `97.
https://www.mate-look.org/p/1313764
https://github.com/matthewmx86/Redmond97
This is a theme that resembles Windows `98. A good theme for a retro desktop look.
CDE / Motif theme.
https://www.mate-look.org/p/1231025
A nice theme that gives a retro UNIX look and feel to the MATE desktop.
This also provides a CDE panel that is best used with the Xfce 4 desktop. This would emulate an old Solaris desktop very well.
CDE is difficult to compile on Linux, so sometimes it is good to fake it.