index.css

:root {
    --main-bg-colour: seashell;
    --textbox-bg-colour: seashell;
    --textbox-border-invalid-colour: crimson;
    --textbox-border-valid-colour: burlywood;
}

* { box-sizing: border-box; }

body {
    font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, Oxygen, Ubuntu, Cantarell, 'Open Sans', 'Helvetica Neue', sans-serif;
    background-color: var(--main-bg-colour);
    padding: 25px;
    display: flex;
    flex-direction: column;
    min-height: 100vh;
}

main {
    display: flex;
    flex-direction: column;
    align-items: center;
    flex: 1;
}

#label {
    font-size: 2rem;
    width: 40%;
    text-align: center;
}

#description {
    margin: 4px 0;
    font-size: 1rem;
}

@media (max-width: 700px) {
    #label {
        width: 60%;
    }
}

@media (max-width: 512px) {
    #label {
        width: 90%;
    }
}

#numbers {
    font-size: 1.5rem;
    display: block;
    width: 100%;
    height: 40px;
    text-align: center;
    background-color: var(--textbox-bg-colour);
    outline: 0;
    color: black;
    margin-top: 20px;
}

#numbers:invalid {
    border: 1px solid var(--textbox-border-invalid-colour);
}

#numbers:valid {
    border: 1px solid var(--textbox-border-valid-colour);
}

#result {
    font-size: 1rem;
    width: 70%;
    padding: 10px;
    text-align: center;
    list-style: none;
}

#loading, #empty {
    font-size: 1rem;
}

footer {
    height: 100px;
    display: flex;
    justify-content: center;
    align-items: center;
}

.fa-github {
    font-size: 2.5em;
    color: rgba(0, 0, 0, 0.7);
}

.fa-github:hover {
    color: rgba(0, 0, 0, 0.8);
}