Classic Grey Theme is designed for Fennec, mobile Firefox, to look more like old classic interface: not smooth, but distinct. Its main goal it to be simple and easy-to-see.
Every button has a distinct border.
Most icons are from default theme: they are simple and non-distracting.
The theme takes seven times less memory on disk, than default theme. It's due to usage of css (text) to describe borders instead of border images and due to simplicity of the images used.
As name indicates, the background colour is mostly light grey.
One difference from the default theme: in portrait mode the search engines have no labels, only icons, for comfort while using multiple (four, five, six, seven) search engines.