invalid deck indicator needs to be a lot more visible than it is right now. i didn't understand what you meant when you said you were gonna do a red border. i was thinking you meant a big red border around the deck, not around one card.
still recommend a big bright red "invalid" that changes to a green "valid" when you make a real deck. probably over by the exit/etc buttons, possibly below buttons and next to mark
also, add short reminder of deckbuilding rules to the editor screen, probably along the bottom under collection