.calculator{
    width: 230px;
    background-color: #000;
    border-radius: 5px;
    padding: 15px;
    display: flex;
    flex-wrap: wrap;
}

h1{
    color: white;
}

#display{
    width: 100%;
    padding: 7px;
    margin-bottom: 10px;
} 

.btn{
    width: 40px;
    height: 40px;
    border-radius: 50%;
    border: 0;
    margin-bottom: 10px;
}