style.css

html, body {
    margin: 0;
    padding: 0;
}

body {
    background: #cccccc;
    color: #171717;
    font-size: 16px;
}

.container {
    padding: 10px;
    background: #ffffff;
    position: absolute;
    left: 0;
    right: 0;
    top: 50%;
    width: 400px;
    margin: 0 auto;
}

.container .captcha-holder {
    margin: 0 auto;
}

.captcha-holder {
    position: relative;
    max-width: 323px;
    padding: 5px;
    padding-bottom: 0;
    border: 1px solid rgba(0, 0, 0, 0.2);
}

.captcha-holder .captcha-title {
    padding: 5px;
    text-align: center;
    font-size: 14px;
    border-bottom: 1px solid rgba(0, 0, 0, 0.2);
}

.captcha-holder .captcha-icons {
    display: flex;
    flex-direction: row;
    width: 100%;
}

.captcha-holder .captcha-icons .captcha-selector {
    position: relative;
    flex-basis: 20%;
    text-align: center;
    padding: 10px 0;
    cursor: pointer;
}

.captcha-holder .captcha-icons .captcha-selector .captcha-icon {
    margin: 0 auto;
}

.captcha-holder .captcha-icons .captcha-selector .captcha-checkbox {
    position: absolute;
    bottom: 0;
    width: 100%;
    height: 3px;
}

.captcha-holder .captcha-selector .captcha-checkbox input[type=checkbox] {
    display: none;
}

.captcha-holder .captcha-selector .captcha-checkbox .check {
    position: absolute;
    width: 100%;
    height: 3px;
    left: 0;
    bottom: 0;
}

.captcha-holder .captcha-selector .captcha-checkbox input[type=checkbox]:checked+span {
    background: dodgerblue;
}