.countdown-container {
    color: #333;
    margin: 0 auto;
    text-align: center;
}

#countdown > ul > li {
    display: inline-block;
    font-size: 1.5em;
    list-style-type: none;
    padding: 1em;
    text-transform: uppercase;
}

#countdown > ul > li span {
    display: block;
    font-size: 4.5rem;
}

@media all and (max-width: 768px) {
    #countdown > h1 {
        font-size: calc(1.5rem * .75);
    }

    #countdown > ul > li {
        font-size: calc(1.125rem * .75);
    }

    #countdown > ul > li span {
        font-size: calc(3.375rem * .75);
    }
}