I'll just leave this here since people don't know better. If you want to custom sleeve you cards you can follow the instruction from Darkdove3 https://youtu.be/51xnDAgY3mY?si=fHJ2NrR_tSDkxzqt
TLDR
1. Go to your TTS Save Object file in your file explorer
2. Open the Json file with the decks name
3. Do Crtl + H to find and replace the BackURL links for all of your cards in the deck
4. To get the art design you want you can just google search them then just copy image link

This mod is still very useful for getting a collection of different card backings by
1. Right clicking a card and clicking on "custom"
2. Grab the link next to "Back"
3. Plop it into Step 3 of the above process