Lucidity was a proposed theme for Ubuntu 10.04 but despite being only a mock-up it proved so popular with users that a Metacity based on it soon sprang forth.
Now a GTK+ theme to match it has been created by themer daysofruin.
The theme comes with two ‘variants’ – a ‘square’ version : -
And a rounder version more akin to the mock-up: -
Lucidity also comes with it’s own Chrome/ium theme. The theme works best when ‘system’ borders are used rather than Chrome/ium’s own.
Note that Ludiity GTK needs the latest version of the Murrine engine. You can grab a .deb for that @ gnome-look.org/Murrine+Engine+Ubuntu+Packages