style.css

* {
    font-family: 'Open Sans', sans-serif;
}

body {
    background-color: #d1d4d6;
    display: flex;
    align-items: center;
    justify-content: center;
    height: 100vh;
    margin: 0 10px;
}

.calculator {
    width: 100%;
    max-width: 300px;
    padding: 30px;
    border-radius: 5px;
    background-color: white;
    box-shadow: 5px 5px 10px grey;
}

#output input {
    background-color: #9c9a9ade;
    display: block;
    width: 100%;
    height: 78px;
    color: white;
    font-size: 40px;
    text-align: right;
    border: 0;
}

.calculator .buttons {
    margin-top: 25px;
    display: grid;
    grid-template-columns: repeat(4,1fr);
    grid-gap: 20px;
}

.calculator .buttons button  {
    font-size: 20px;
    background-color: #efefef;
    border: 1px solid #d4d2d2;
    cursor: pointer;
    height: 50px;
}

.calculator .buttons button:active{
    background-color: #aaa9a9;
}

.calculator .buttons button:focus{
    outline: 0;
}

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