style.css

* {
    margin: 0;
    padding: 0;
}
.fundo {
    background-image: linear-gradient(45deg,rgb(0, 0, 0), rgb(165, 151, 178));
    height: 100vh;
    color: #fff;
    font-family: Arial, Helvetica, sans-serif;
    text-align: center;
}

.calculadora {
    position: absolute;
    background-color: rgb(0, 0, 0, 0.8);
    top: 50%;
    left: 50%;
    transform: translate(-50%, -50%);
    border-radius: 15px;
    padding: 15px;
}

#resultado {
    background-color: white;
    width: 207px;
    height: 30px;
    margin: 5px;
    font-size: 25px;
    color:black;
    text-align: right;
    padding: 5px;
}

.botao {
    background-color: rgb(52, 51, 51);
    height: 50px;
    width: 50px;
    font-size: 25px;
    cursor: pointer;
    border: none;
    color: #fff;
    margin: 3px;
}
.botao:hover {
    background-color: black;
}