.modal-prompt {
    background: #fff;
    border-radius: 2px;
    box-shadow: 0px 0px 10px rgba(0,0,0,0.5);
    width: 80%;
    margin: 0 auto;
    position: relative;
    overflow: hidden;
    z-index: 1000000;
    top: 20%;
    height: 60%;
    max-width: 640px;
    max-height: 640px;
}
.modal-prompt p, .modal-prompt h1 {
    color: #555555;
}
#prompt-background {
    background: url("https://s3.amazonaws.com/registration.flowid.me/images/banner_dark.jpg") no-repeat center center fixed #555;
    -webkit-background-size: cover;
    -moz-background-size: cover;
    -o-background-size: cover;
    background-size: cover;
    width: 100%;
    height: 100%;
    position: fixed;
    left: 0;
    top: 0;
    z-index: 9999999;
}

.modal-prompt select { float: left; margin-right: 10px; } 

/* Source: https://www.w3schools.com/howto/howto_css_responsive_iframes.asp */
.responsive-iframe {
    position: absolute;
    top: 0;
    left: 0;
    bottom: 0;
    right: 0;
    width: 100%;
    height: 100%;
  }