One of the great things about the Ubuntu Mate GTK2 distro, is you get to use all the great library of GTK2 themes that have been around forever. I've always liked the brushed metal look. The Tish Brushed Theme fills the bill nicely. See below and where you can get this oldie but goodie MAC like brushed metal theme for Ubuntu Mate GTK2. Enjoy.
You can get the Tish Brushed Metal theme here:
https://gnome-look.org/content/show.php?content=30972&forumpage=2Another nice brushed metal theme is GTBrushed here:
http://gnome-look.org/content/show.php/GTBrushed?content=78861