style.css

* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
}

html{
    font-size: 10px;
}

body{
    font-size: 1.2rem;
}

.calculator{
    border: 2px solid #c4c4c4;
    border-radius: 20px;
    width: 300px;
    position: absolute;
    top: 50%;
    left: 50%;
    transform: translate(-50%,-50%);
}

.calculator-input{
    width: 100%;
    font-size: 4rem;
    height: 50px;
    border: 1.5px solid #c4c4c4;
    border-radius: 15px;
    background-color: transparent;
    color: black;
    text-align: right;
    padding-left: 20px;
    padding-right: 20px;
}

button{
    height: 50px;
    background-color: #fff;
    border-radius: 10px;
    border: 1.5px solid #c4c4c4;
    background-color: transparent;
    font-size: 3rem;
    color: #333;
    cursor: pointer;
    transition: 0.8s;
}

button :hover{
    background-color: cadetblue;
    color: cadetblue;
    height: 55px;
}

.operator{
    color: #337cac;
}

.clear{
    background-color: #e77f82;
    border-color: #d65e62;
    color: #fff;
}

.clear :hover{
    background-color: #a31f23;
}

.equal-sign{
    background-color: #64addf;
    border-color: #337cac;
    color: #fff;
    height: 100%;
    grid-area: 2/4/6/5;
}

.equal-sign :hover{
    background-color: #214c69;
}

.calculator-keys{
    display: grid;
    grid-template-columns: repeat(4,1fr);
    grid-gap: 20px;
    padding: 20px;
    
}