The CSS file loaded from here:
/wp-content/plugins/s2member/s2member-o.php?ws_plugin__s2member_css=1&qcABC=1&ver=170722-170722-1767684734
References images by HTTP URL, eg:
label.s2member-pro-paypal-form-card-type-visa-label {
background: url(‘http://domain.com/wp-content/plugins/s2member-pro/src/images/visa-card.png’) no-repeat top center
}
All WP settings are set to HTTPS and s2member_force_ssl is set to yes on the page.