#calculator {
    width: 300px;
    margin: auto;
    padding: 10px;
    border: 1px solid black;
}

#screen {
    width: 100%;
    padding: 5px;
    margin-bottom: 10px;
    font-size: 18px;
}

button {
    width: 100%;
    height: 40px;
    font-size: 18px;
    margin-bottom: 5px;
    background-color: #EEE;
    border: 1px solid #DDD;
    border-radius: 5px;
}

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