body{
    width: 43%;
    margin: auto;
}
h1{
    text-align: center;
    font-size: 2.2rem;
    font-family: Verdana, Geneva, Tahoma, sans-serif;
}
#res{
    background-color: lightgray;
    border: solid;
    height: 48px;
    font-size: 40px;
}
#btn0,#btn1{
    background-color: lightgreen;
    color: brown;
}
#btnClr,#btnEql{
    background-color: darkgreen;
    color: white;
}
#btnSum,#btnSub,#btnMul,#btnDiv{
    background-color: black;
    color: red;
}
button{
    width: 25%;
    height: 56px;
    font-size: 28px;
    margin: 0px;
    float: left;
}

@media only screen and (max-width: 600px) {
    body{
        width: 80%;
    }
}