Some awesome themes for the Linux Mint 15 MATE desktop.
Numix: http://satya164.deviantart.com/art/Numix-GTK3-theme-360223962. This is a very nice flat theme for MATE that is very monochome and stylish.
Clearlooks Phenix: http://www.jpfleury.net/en/software/clearlooks-phenix.php. This is a GTK 3 port of the venerable Clearlooks theme.
Holo: http://tiheum.deviantart.com/art/Holo-280076980. This is a very good theme that mimics the look of the Google Android theme.
Ice Cream Sandwich GTK theme: http://tmari0.deviantart.com/art/Ice-Cream-GTK-277483456.[caption id="" align="aligncenter" width="400"] Ice Cream Sandwich GTK theme.[/caption]
MintLiner GTK 3 theme: http://danodymake.deviantart.com/art/MintLiner-gtk3-theme-207558801. A beautiful GTK theme that is perfect for a Linux Mint user.
Autumn GTK theme: http://sizakor.deviantart.com/art/Autumn-gtk-3-gtk-2-theme-324042087. This theme resembles the Ubuntu Ambience theme. Very nice theme too.
Full flat GTK theme: http://lgsalvati.deviantart.com/art/Full-flat-GTK3-theme-0-2-255255056. Bueno! This theme is cool.
Siva Flat 1.3.0 GTK theme: http://nale12.deviantart.com/art/Siva-Flat-1-3-0-353499016. Another very good flat GTK theme for Gnome Shell or MATE.