:root {
    --box-width: min(400px, calc(100% - 100px));
    --button-space: 22px;
}

* {
    box-sizing: border-box;
}

body {
    color: white;
    font-family: Ubuntu, sans-serif;

    background-image: url("cat-background.png");
    background-position: center bottom;
    background-attachment: fixed;
    background-repeat: no-repeat;
    background-size: cover;
}

.box {
    width: var(--box-width);
    margin: 50px auto;
    padding: var(--button-space);

    background-color: #171728;
    border-radius: 16px;
    text-align: center;
}

button {
    width: 100%;
    padding: 11px;
    margin-top: var(--button-space);

    background-color: mediumpurple;
    border: none;
    border-radius: 8px;

    font-size: 20px;
    color: white;

    cursor: pointer;
    transition: background-color 0.3s ease;
}

button:hover {
    background-color: rebeccapurple;
}

button:active {
    background-color: rebeccapurple;
}

.back-button {
    width: 80px;
    opacity: 80%;
    transition: opacity 0.3s ease;

    position: absolute;
    top: 10px;
    left: 10px;
}

.back-button:hover {
    opacity: 100%;
}

.back-button:active {
    opacity: 60%;
}

.unselectable {
    -webkit-user-drag: none;
    user-select: none;
    -moz-user-select: none;
    -webkit-user-select: none;
    -ms-user-select: none;
}

@media (max-width: 600px) {
    .back-button {
        position: relative;
    }
}