The paperwork
Credits
Card faces
Vector Playing Cards 3.2 by Chris Aguilar, licensed under the LGPL 3.0. The full license and author list ship with the site: LICENSE.txt · AUTHORS.txt.
Type
Display face: Russo One, licensed under the SIL Open Font License.
Sound
Every sound in the game is synthesized in code; there are no recordings.
Everything else
The rival faces, the card back, the stamps and the felt are drawn by the game's own painters. Nothing generated, nothing stock.