Good Concept! I wish you could double click on the cards in your hand, rather than on the left, and I wish that you'd be able to get more than 1 card back when you put more than 1 white card in. no matter if i put in 1 or 12, i'd always get 1 blue, pink, or yellow card back.
I'd love some pitch variety in the SFX and maybe some background music.
I think the implementation of the theme is a little subpar, since you're not truly invincible, you're just giving yourself extra health through the shield, rather than complete invulnerability.
I like the card mechanics, and I think there's some good room for improvement and polish.
Good job on submitting a project in time though!