index.css

/* -- Utility Classes -- */

.hidden {
    display: none !important;
}

/* -- General Styles -- */

body {
    margin: 20px;
    font-family: Times, "Times New Roman", Georgia, serif;
}

h1, h2, h3, h4, h5, h6 {
    font-family: Verdana, Arial, Helvetica, sans-serif;
}

#widget {
    max-width: 600px;
    margin: 0 auto;
}

/* -- Form Styles -- */

form * {
    display: block;
}

form label {
    font-size: 16px;
    font-family: Verdana, Arial, Helvetica, sans-serif;
    padding-bottom: 5px;
}

form input#ccNumber {
    width: 100%;
    font-size: 24px;
    border: none;
    background-color: #eeeeee;
}

form span.validation {
    margin-bottom: 15px;
    min-height: 18px;
    color: #E0115F;
    font-style: italic;
}

form button {
    font-size: 18px;
    padding: 6px 20px;
    border: none;
    background-color: #eeeeee;
    color: #000000;
    float: right;
    cursor: pointer;
    margin-left: 10px
}

form button[type=submit] {
    background-color: #00834E;
    color: #ffffff;
}

form button.inactive {
    background-color: #aaaaaa;
    color: #000000;
}

.issuers span {
    filter: grayscale(100%);
    height: 48px;
    width: 68px;
    display: inline-block;
    padding: 3px;
    margin-right: 5px;
    border: 1px solid #000000;
    border-radius: 5px;
    background-size: contain;
    background-repeat: no-repeat;
    background-position: center;
    background-origin: content-box;
    box-sizing: border-box;
}

.issuers span.active {
    filter: none;
}

.issuers .visa {
    background-image: url('images/visa.png');
}

.issuers .mc {
    background-image: url('images/mc.png');
}

.issuers .discover {
    background-image: url('images/discover.png');
}

.result {
    margin-top: 40px;
}

.resultText {
    font-size: 24px;
    text-align: center;
    color: #00834E;
    font-family: Verdana, Arial, Helvetica, sans-serif;
}

.error .resultText {
    color: #E0115F;
}