styles.css

*, *::before, *::after {
    box-sizing: border-box;
    font-family: 'Open Sans', sans-serif;
    font-weight: normal;
}

body {
    padding: 0;
    margin: 0;
    background: linear-gradient(to left, grey, black);
}

.container-grid {
    display: grid;
    justify-content: center;
    align-content: center;
    min-height: 100vh;
    grid-template-columns: repeat(4, 80px);
    grid-template-rows: minmax(120px, auto) repeat(5, 80px);
}

.container-grid > button {
    cursor: pointer;
    font-size: 1.5rem;
    border: 1px solid white;
    outline: none;
    background-color: rgba(255,255,255,0.75);
}

.container-grid > button:hover {
    background-color: rgba(255,255,255,0.90);
}

.span-two {
    grid-column: span 2;
}

.output-screen {
    grid-column: 1/-1;
    background-color: rgba(0,0,0,0.75);
    display: flex;
    align-items: flex-end;
    justify-content: space-around;
    flex-direction: column;
    padding: 15px;
    word-wrap: break-word;
    word-break: break-all;
}

.output-screen .previous-operand {
    color: rgba(255,255,255,0.70);
    font-size: 1.5rem;
}

.output-screen .current-operand {
    color: white;
    font-size: 2.3rem;
}