A bit about the technology behind GNOME Shell.
The Shell is using OpenGL graphics. It's built on top of Clutter, which is a scene graph that uses OpenGL for rendering. For the window management and compositing, the Shell uses Mutter window manager, which is a branch of the traditional window manager for GNOME called Metacity that includes Clutter-based compositing.
We have a Shell Toolkit on top of Clutter, which implements many custom widgets, such as boxes that help with the layout, buttons or scrollbars. The Shell Toolkit also adds support for CSS, which makes the Shell easily themeable.