Ubuntu re-merge
Preparing the forthcoming Lubuntu 15.04 Vivid Vervet release, the Box theme has been updated and re-merged with Ubuntu’s Light Theme to the use last GTK features, like the new HeaderBars used in Gnome native apps. Changes include a lot of bug fixes, adjustments in colour definitions, modified headerbars and toolbars, improved paddings and distances, improved 3rd party apps support (like Nautilus, Maps or Music), etc.
If you want to use Box theme in your distro just download it from the Artwork page.