style.css

*, *::before, *::after{
    box-sizing: border-box;
    font-family: 'Segoe UI', Tahoma, Geneva, Verdana, sans-serif;
    font-weight: bold; 
}

body{
    padding: 0;
    margin: 0;
    background: darkolivegreen;
    
}

.calc-grid{
    display: grid;
    justify-content: center;
    align-content: center;
    min-height: 100vh;
    position: relative;
    grid-template-columns: repeat(4, 150px);
    grid-template-rows: minmax(150px, auto) repeat(5, 150px);
}

.calc-grid > button{
    cursor: pointer;
    font-size: 2rem;
    border: 1px solid white;
    border-radius: 15%;
    outline: none;
    background-color: rgba(62, 86, 218, .75);
}

.calc-grid > button:hover{
    background-color:  rgba(157, 168, 229, 0.90);
}

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

.output{
    grid-column: 1 / -1;
    background: darkslategray;
    display: flex;
    justify-content: space-around;
    align-items: flex-end;
    flex-direction: column;
    border: 1px solid white;
    border-radius: 5%;
    padding: 10px;
    word-wrap: break-word;
    word-break: break-all;
}

.output .prev-operand{
    color: rgba(255, 255, 255, .75);
    font-size: 1.5rem;
}

.output .cur-operand{
    color: white;
    font-size: 2.5rem;
}