GTK MATE theme: http://gnome-look.org/content/show.php/CDE-Solaris?content=19627. This is a lovely grey theme that themes the GTK interface to look like CDE.
The Metacity theme that completes the CDE look: http://gnome-look.org/content/show.php?content=18870. This is perfect if you want a retro looking Linux Mint 15 MATE desktop.
Blue Sun GTK theme: http://gnome-look.org/content/show.php/Blue+Sun+GTK+Theme?content=93650. This is a very dark and attractive GTK theme.
Another cool CDE styled GTK theme: http://gnome-look.org/content/show.php/Murrina+Solaris?content=92982.
Gray Shiki-Wise: http://gnome-look.org/content/show.php?content=124949. A gorgeous and light GTK theme.
Another interesting Solaris Motif theme: http://gnome-look.org/content/show.php?content=18118. This theme is another one that gives a Solaris look to your Linux Mint 15 MATE desktop.
RiscOS GTK theme: http://gnome-look.org/content/show.php/?content=128771. This is the definitive GTK theme that looks just like the old RiscOS style.
There are a few lovely GTK themes here: http://art.gnome.org/themes/gtk2. These themes are incredible.
BeOS GTK theme: http://gnome-look.org/content/show.php/BeOS?content=69. This is a GTK 1 theme. To use it with MATE, go to the ~/.themes/BeOS folder and type ln -s gtk gtk-2.0. Then the MATE Appearance dialog will let you select the theme.
homer@deep-thought ~/.themes/BeOS $ ls -hula total 24K drwxr-xr-x 4 homer homer 4.0K Jun 12 20:26 . drwx------ 112 homer homer 4.0K Jun 12 20:25 .. drwxr-xr-x 2 homer homer 4.0K Jun 12 20:26 gtk drwxr-xr-x 2 homer homer 4.0K Jun 12 20:26 gtk-2.0 -rw-r--r-- 1 homer homer 3.5K Jan 1 1970 ICON.png -rw-r--r-- 1 homer homer 593 Jan 1 1970 README.html
There is another good BeOS theme here though: http://www.zevenos.com/wp-content/uploads/2009/11/Haiku.tar.gz. This theme works perfectly with MATE.