styles.css

* {
    box-sizing: border-box;
    font-family: Tahoma, sans-serif;
    font-weight: bold;
}

body{
    margin: 0;
    padding: 0;
    background: linear-gradient(to right, #D1D0D0, #656565);
    
}

.calculator-container{
    margin:auto;
    padding:0;
    width:100%;
    height: 100vh;
    display: flex;
    justify-content: center;
    align-items: center;
}

.calculator-grid {
    display: grid;
    justify-content: center;
    align-content: center;
    height: 600px;
    padding: 60px 18px 0px 18px;
    background-color: #080808ef;
    border-radius: 50px;
    grid-template-columns: repeat(4,70px);
    grid-template-rows: 100px repeat(5,70px);
}

.calculator-grid > button{
    font-size: 1.3rem;
    border-radius: 50%;
    background-color: #656565;
    border: none;
    width: 90%;
    height:90%;
}

.calculator-grid > button.top-level{
    background-color: #f100007e;
}

.calculator-grid > button.right-side{
    background-color: #353535ef;
    color: #e4e4e4ef;
}

.calculator-grid > button:hover{
    background-color: #ababab;
    cursor: pointer;
}

.calculator-grid >button:active{
    background-color: #656565;
}

.calculator-grid > button.span-two{
    grid-column: span 2;
    width: 95%;
    border-radius: 80px;
}

.output{
    font-size: 1rem;
    display: flex;
    grid-column: span 4;
    overflow: hidden;
    align-items: flex-end;
    justify-content: space-between;
    flex-direction: column;
    background-color: rgb(214, 212, 212);
    border-radius: 10px;
    height: 85%;
    padding: 10px 15px 10px 0px;
}

.output > .previous-operand{
    color: #353535ef;
    font-size: 1.2em;
}

.output > .current-operand{
    color:#080808de;
    font-size: 1.8em;
}