style.css

*,*::before, *::after {
    box-sizing: border-box;
    font-family:Gotham Rounded, sans-seriff;
    font-weight: normal;
}
body {
    pading :0;
    margin: 0;
    background:linear-gradient(to right,#00AAFF,#00FF6C);
}
.calculator-grid {
    display: grid;
    justify-content : center;
    align-content:center;
    min-height : 100vh;
    grid-template-columns: repeat(4,100px);
    grid-template-rows: minmax(120px,auto) repeat(5,100px);
}
.calculator-grid > button {
    cursor:pointer;
    font-size:2rem;
    border: 1px solid white;
    outline: none;
    background-color: rgba(255,255,255,.75);
}
.calculator-grid > button:hover {
    background-color: rgba(255,255,255,.9);
}
.span-two {
    grid-column: span 2;
}
.output {
    grid-column: 1 / -1;
    background-color: rgba(0,0,0,.75);
    display: flex;
    align-items:flex-end;
    justify-content: space-around;
    flex-direction: column;
    padding :10px;
    word-wrap:break-word;
    word-break: break-all; 
}
.output .previous-operand {
    color:rgba(255,255,255,.75);
    font-size: 1.5rem;
}
.output .current-operand {
    color:white;
    font-size:2.5rem;
}