The Card Back feature allows you to alter the card design. You can unlock new card backs by completing certain requirements, such as completing quests or winning five games in a ranked play season. Some cards are also available for purchase in the game store for 500 gold, but this doesn’t relate to most of the limited edition designs, with rare exclusions. You can only select the card backs from the suggested cards, so creating a custom design is impossible. To view the available card backs, navigate to the “My Collection” menu. Then, click the gear icon in the upper-right corner of your screen and select the card back you prefer.