First of all in Linux there are different types of themes:
1. Pixmap themes, based on the pixbuf engine.
2. Themes based on various engines, the most popular are clearlooks, ubuntulooks, smooth, glider, industrial, and of course Murrine.
Murrine is an engine, written in C language, using cairo vectorial drawing library to draw widgets.
Then you need to install themes in order to see what murrine can do! :)