The current theme is Carved-out or Imprinted. This means letters that have been made by carving out or imprinting-compressing whatever it is they're made of. For instance, this A has been carved out, and the B has been imprinted:
Haha, nice speed - but not very democratic. I'm thinking that with the next them we should also announce the theme after, in order to give more people a chance to collect targeted letters. Otherwise it'll just be peple who already have large collections and/or are in the habit of daily photo walks.