body {
    background-image: linear-gradient(rgb(0, 0, 0), rgba(0, 0, 0, 0.500));
    height: 100dvh;
    display: flex;
    flex-direction: column;
    justify-content: center;
    align-items: center;
}

#birthday-date {
    font-size: 1.4rem;
    padding: 1.2rem;
    text-align: center;
}

button {
    margin-top: .5rem;
    padding: 1.8rem;
    font-size: 1.4rem;
}

.result {
    min-height: .8rem;
}

.text-result {
    color: #fff;
    font-weight: bold;
    font-family: sans-serif, 'Courier New', Courier, monospace;
    font-size: 1.8rem;
    text-shadow: 0 0 10px #f00;
}

/* removendo as "setas" do input que é do tipo NUMBER. */
/* Para navegadores baseados no WebKit (Chrome, Safari) */
input[type="number"]::-webkit-inner-spin-button,
input[type="number"]::-webkit-outer-spin-button {
    -webkit-appearance: none;
    margin: 0;
    text-align: center;
}