style.css

@import url('https://fonts.googleapis.com/css2?family=Poppins:wght@200;300;400;500;600;700&display=swap');

*{
    margin: 0;
    padding: 0;
    font-family: 'Poppins',sans-serif;
}
body{
    height: 100vh;;
    width: 100%;
    background-image: linear-gradient(#C9D6FF, #E2E2E2 100%);
}
.main-body{
    margin-left: 50ch;
    margin-top: 100px;
}
.calc{
    width: 400px;
    height: 470px;
    background: linear-gradient(#232526,#414345);
    border-radius: 5%;
}
#screen{
    height: 60px;
    width: 90%;
    pointer-events: none;
    margin-left: 3.5%;
    margin-top: 30px;
    font-size: 30px;
}
.btn{
    width: 65px;
    height: 45px;
    margin-left: 24px;
    margin-top: 20px;
    background: #fff;
    color: #000;
    font-size: large;
}
.btn:hover{
    background: #000;
    color: #fff;
}
input[type=text]{
    background: #696969;
    color: #fff;
    padding: 3px;
}

.btn-row{
    width: 345px;
}