Don't like boring, default grey? Me neither! Here's how to set your Cover Card on your deck.
First, enter the Deck Editing mode, which is where you're able to add/remove cards from your deck.
From here, click on the card you want to make the cover from your deck list on the right. On mobile, click the yellow up arrow to view your deck list.
This will open the card details modal. In here, click the three dots menu and then click Make Cover Card.
You can show either an Art Focus or Full Card version for your deck cover card. Access your Deck Settings to set this to whichever you prefer.
Full Card will show the entire card as your deck cover. Art Focus will zoom your cover card into the art portion of the card.