.back{
    display: flex;
    justify-content: center;
    
}

section{
    margin-top: 10px;
    background: rgba(179,220,237,1);
    
    -webkit-box-shadow: 10px 10px 35px 0px rgba(51,51,51,1);
-moz-box-shadow: 10px 10px 35px 0px rgba(51,51,51,1);
box-shadow: 10px 10px 35px 0px rgba(51,51,51,1);
background: -moz-linear-gradient(-45deg, rgba(179,220,237,1) 0%, rgba(41,184,229,1) 50%, rgba(188,224,238,1) 100%);
background: -webkit-gradient(left top, right bottom, color-stop(0%, rgba(179,220,237,1)), color-stop(50%, rgba(41,184,229,1)), color-stop(100%, rgba(188,224,238,1)));
background: -webkit-linear-gradient(-45deg, rgba(179,220,237,1) 0%, rgba(41,184,229,1) 50%, rgba(188,224,238,1) 100%);
background: -o-linear-gradient(-45deg, rgba(179,220,237,1) 0%, rgba(41,184,229,1) 50%, rgba(188,224,238,1) 100%);
background: -ms-linear-gradient(-45deg, rgba(179,220,237,1) 0%, rgba(41,184,229,1) 50%, rgba(188,224,238,1) 100%);
background: linear-gradient(135deg, rgba(179,220,237,1) 0%, rgba(41,184,229,1) 50%, rgba(188,224,238,1) 100%);
filter: progid:DXImageTransform.Microsoft.gradient( startColorstr='#b3dced', endColorstr='#bce0ee', GradientType=1 );
    display: flex;
    flex-direction: column;
    justify-content: center;
    align-items: center;
    padding: 20px;
    border-radius: 30px;
    outline: 15px double #eee
}
.header-div{
    display: flex;
    justify-content: center;
    align-items: center;
}
.div-calculator{
    display: flex;
    justify-content: center;
    align-items: center;
}

.calculator-window{
    z-index: 100;
    display: flex;
    justify-content: center;
    flex-direction: column;
    width: 250px;
    height: 100px;
    border: 3px solid #333;
    border-radius: 20px;
    background-color: #eee;
}


.characters{
    display: grid;
    grid-template-columns: repeat(4, 50px);
    grid-template-rows: repeat(4, 40px);
    margin: 10px;
    padding: 20px;
    justify-content: center;
}

button{
    padding: 10px;
    text-align: center;
    margin: 1px;
    border-radius: 10px;
    background-color: eee;
}

button:hover{
    background-color: #ccc;
}

.operators{
    background-color: aliceblue;
}