I was happy to see the Witcher-themed cards get added in the latest update, but was disappointed when I learned it only changes the Spade face cards. So my friend undertook the task of filling in the remaining face cards, aces and remaining cards.
Links to his Tumblr[www.tumblr.com]
And his Steam

For more details look at the "Replacing the original deck of cards with your own section" of this guide.

In short:

• Open Balatro.exe in a archive tool like 7zip
  
• Navigate to \resources\textures\2x\
  
• Replace 8BitDeck.png with one of the following images. (IT MUST BE NAMED 8BitDeck.png)