Actually Nostromo is made by us from scratch (a quick search on Google will also reveal that at this time there’s no public theme called Nostromo anywhere; you could’ve also looked deeper into the code and seen comments, descriptions etc. by us).
It’s just that this time we decided to create a theme specifically for our own site and our very specific needs. Also, before creating Nostromo we have been using our own theme Mantra for over 6 years on this very site.
Anyway, if you like the look of our new site you’ll be glad to know we’ll be releasing a theme based on it in the near future.