body {
    font-family: 'Source Sans Pro', 'Helvetica Neue', Helvetica, Arial, sans-serif;
    background-color: black;
    color: white;
    width: 100%;
    max-width: 500px;
}

main {
    display: grid;
    grid-template-columns: 1fr 1fr 1fr 1fr;
    grid-column-gap: 10px;
    grid-row-gap: 10px;
}

button {
    padding: 0;
    cursor: pointer;
    text-align: center;
    vertical-align: middle;
    border: none;
    border-radius: 50%;
    width: 20vh;
    height: 20vh;
    max-height: 20vw;
    max-width: 20vw;
    font-size: 6vh;
    touch-action: manipulation;
}

.operator {
    background-color: rgb(240, 154, 57);
    color: white;
}

.special,
.clear,
.backspace {
    background-color: rgb(165, 165, 165);
    color: black;
}

.number {
    background-color: rgb(51, 51, 51);
    color: white;
}

.output {
    height: 1rem;
    text-align: right;
    padding-bottom: 15px;
}

.previous {
    text-align: right;
    height: 1em;
}

h1 {
    text-align: center;
}