style.css

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

body{
    background: linear-gradient(to right, #63e2ff, #00ff88);
}

.calculator {
    text-align: center;
    margin: 50px auto;
    width: 400px;
    background-color: white;
    border-radius: 25px;
}

.heading{
    background-color: rgb(0, 0, 0);
    color: white;
    border-top-left-radius: 25px;
    border-top-right-radius: 25px;
    font-size: 35px;
    padding: 5px;
}

form{
    align-items: center;
    line-height: 40px;
    font-size: 25px;
    padding: 15px;
}

button{
    background-color: rgb(255, 99, 71);
    color: rgb(245, 222, 179);
    box-shadow: none;
    width: 80px;
    height: 40px;
    border-radius: 5px;
}

button:hover{
    background-color: rgb(255, 85, 55);
}

input{
    width: 60%;
    height: 35px;
    margin-bottom: 15px;
}

select{
    width: 60%;
    height: 35px;
    margin-bottom: 15px;
}

#total{
    align-items: center;
    display: none;
    font-size: 25px;
}