Upcoming filter in PMPro v 3.1.2 or higher (not currently released) to adjust the appearance of Stripe Card Element styles.

my_custom_pmpro_stripe_card_element_style.php

<?php
/**
 * Upcoming filter in PMPro v 3.1.2 or higher (not currently released) to adjust the appearance of Stripe Card Element styles.
 * Refer to this Stripe documentation for available properties: https://docs.stripe.com/js/appendix/style
 */
function my_custom_pmpro_stripe_card_element_style($styles) {
    $custom_styles = array(
        'base' => array(
            'color' => '#FFFFFF',
            '::placeholder' => array(
                'color' => '#FFFFFF'
            )
        )
    );

    return array_merge($styles, $custom_styles);
}
add_filter('pmpro_stripe_card_element_style', 'my_custom_pmpro_stripe_card_element_style');