body {
    font-family: "Courier New", sans-serif;
    text-align: center;
}
.buttons {
    font-size: 4em;
    display: flex;
    justify-content: center;
}
.button, .value {
    line-height: 1;
    padding: 2rem;
    margin: 2rem;
    border: medium solid;
    min-height: 1em;
    min-width: 1em;
}
.button {
    cursor: pointer;
    user-select: none;
}
.minus {
    color: red;
}
.plus {
    color: green;
}
.value {
    min-width: 2em;
}
.state {
    font-size: 2em;
}