body {
    text-align: center;
    flex-direction: column;
    min-height: 100vh;
    margin: 0;
    background-color: #111;
    color: #eee;
}

header {
    flex: 0 0 auto;
}

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

}

.calculator {
    display: grid;
    grid-template-columns: repeat(4, 1fr);
    grid-template-rows: auto;
    grid-gap: 5px;
    background: purple;
    padding: 10px;
}

.screen {
    grid-column: 1 / span 4;
    height: 100px;
    background: #eee;
}

button {
    height: 100px;
    width: 100px;
    font-size: 50px
}

footer {
    flex: 0 0 auto;
}