Capsula is a GTK theme born from mashing up Murrine and Clearlooks features.
The theme was largely inspired by OSX Leopard but keeping in mind gtk engines capabilities and style.
You need Murrine svn version that you can obtain and install with commands like follows:
svn co http://svn.gnome.org/svn/murrine/trunk ./murrine-svn
GNOME is an international effort to build a complete desktop environment-the graphical user interface which sits on top of a computer operating system-entirely from free software. This goal includes creating software development frameworks, selecting application software for the desktop, and working on the programs which manage application launching, file handling, and window and task management.
GNOME is part of the GNU Project and can be used with various Unix-like operating systems, most notably Linux, and as part of Java Desktop System in Solaris.
The name originally stood for GNU Network Object Model Environment, though this acronym is deprecated. The GNOME project puts heavy emphasis on simplicity, usability, and making things "just work".