#example {
    width: 100%;
    max-width: 600px;
    margin: 20px auto;
}

#example pre {
    background: #f5f5f5;
    padding: 10px;
}

button {
    cursor: pointer;
    border: 1px solid #ccc;
    padding: 6px;
    border-radius: 2px;
    margin: 0px;
    background: white;
    transition: background 0.3s;
    outline: none;
}
button:hover {
    background: #ccc;
}

button.active {
    background: #d9edff;
}

button.disabled {
    opacity: 0.4;
    cursor: auto;
}

.sep {
    margin: 15px;
}

.current-item {
    position: relative;
}

.current-item:after {
    position: absolute;
    content: '';
    border: 2px solid #d9edff;
    left: -25px;
    height: 100%;
    top: 0;
}