style.css

* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
}

body {
    font-family: Arial, Helvetica, sans-serif;
    background: linear-gradient(240deg,
            rgba(0, 0, 0, 1) 27%,
            rgba(0, 212, 255, 1) 91%);
    color: #fff;
    overflow: hidden;
}

section {
    height: 100vh;
    display: flex;
    flex-direction: column;
    justify-content: center;
    align-items: center;
    text-align: center;
}

.center {
    border: 2px solid #fff;
    background: rgb(19, 190, 194);
    background: linear-gradient(60deg,
            rgba(19, 190, 194, 1) 41%,
            rgba(0, 0, 0, 1) 93%);
    border-radius: 6px;
    padding: 2rem 4rem;
}

h1 {
    text-transform: uppercase;
    color: #fff;
    margin-bottom: 2rem;
    user-select: none;
}

input {
    width: 80%;
    padding: 1rem;
    text-align: center;
    font-size: large;
    margin-bottom: 1rem;
}

/* buttons */

button {
    padding: 0.6rem 2rem;
    margin: 1rem;
    color: #fff;
    background-color: #000;
    border-radius: 5px;
    text-transform: capitalize;
    border: none;
}

button:hover {
    background-color: #222;
}

button:active {
    transform: scale(0.9);
}

.add {
    background: green;
}

.add:hover {
    background: rgba(0, 128, 0, 0.781);
}

.sub {
    background: red;
}

.sub:hover {
    background: rgba(255, 0, 0, 0.822);
}

footer {
    padding: 0.4rem 2rem;
    background-color: #000;
    position: fixed;
    inset: auto 0 0 0;
}

footer a {
    color: #fff;
}