What is a GTK engine:
GTK engines, it's referring to the fundamental widget styling. That is, whether the buttons are flat, or round. Does the progress bar contain animated stripes? Are the corners round or straight. That sort of thing. Then, people release themes based on those engines

Overview:
"Murrine" is an Italian word meaning the glass artworks done by Venicians glass blowers. They're absolutely wonderful and colorful. Murrine has this object to provide the ability to make your desktop look like a "Murrina", which is the Italian singular of the name "Murrine". The Engine is cairo-based, and it's very fast compared to clearlooks-cairo and ubuntulooks (30% faster and more), since Cimitan has tryed to optimize the code and removed a lot of slow gradients to provide this unique style.
http://cimi.netsons.org/pages/murrine.php

Ubuntu deb packages:
I have created a couple of deb packages for ubuntu Dapper and Edgy. Here is were you can download them from:
Dapper Drake
Edgy Eft

Themes:
And themes by Cimitan can be found here:
http://cimi.netsons.org/pages/murrine/themes.php

Additional themes can be found at:
http://www.gnome-look.org/
http://www.xfce-look.org/
by searching "Murrine" or "Murrina"

Steps to getting Murrine running
1) Install the murrine engine
2) Download a theme from http://cimi.netsons.org/pages/murrine/themes.php or http://www.gnome-look.org/ extract the theme/s into ~/.themes
3) Download and install the murrine configurator http://cimi.netsons.org/pages/murrine/configurator.php then after the installation the program can be found under(in Gnome) System -> Preferences -> Murrine Configurator