Stripe's CardElement is self-contained, so we can just add it to the PlaceOrder component, then add styles as desired, and the card detail input is taken care of.
mern-marketplace/client/cart/PlaceOrder.js:
<CardElement className={classes.StripeElement} {...{style: { base: { color: '#424770', letterSpacing: '0.025em', '::placeholder': { color: '#aab7c4', }, }, invalid: { color: '#9e2146', }, }}}/>