many in the crowd will know this already, but the Apple Lisa was a high-end machine aimed at developers, which existed during the transition period between the Apple ][ and the Macintosh. it had many of the core UI metaphors and idioms of the Macintosh, but was just a bit less... there. the first versions of the Macintosh system software were written on the Lisa.
so what you're seeing here is a re-creation of the Lisa's slightly different visual style, as a skin for Mac windows