#marco {
    position:absolute;
    left:200px;
    top:100px;
    width:300px;
    height:400px;
    background-color:black;
    border:3px solid black;
}

#pantalla {
    position:absolute;
    left:225px;
    top:150px;
    width:250px;
    height:60px;
    background-color:white;
    border:3px solid white;
}

#botones {
    position:absolute;
    left:5px;
    top:15px;
    width:150px;
}

#numeros {
    position:absolute;
    left:225px;
    top:250px;
    width:150px;
    height:150px;
    background-color:black;
    border:3px solid black;
}

#operaciones {
    position:absolute;
    left:375px;
    top:250px;
    width:100px;
    height:150px;
    background-color:black;
    border:3px solid black;
} 

button{
    border:none;
    color:dark-gray;
    padding:10px 15px;
    text-align:center;
    font-size:15px;
    margin:4px 2px;
    cursor:pointer;
    display:inline-block
} 
 
h3 {
    margin:3px 3px;
    text-align:right;
}