style.css

@import url('https://fonts.googleapis.com/css2?family=Poppins:wght@100;200;300;400;500;600;700;800&display=swap');

* {
    margin: 0;
    padding: 0;
    font-family: Poppins;
}

.fundo {
    text-align: center;
    background-color: rgb(231, 231, 231);
    height: 100vh;
    color: #2e2b2b;
    font-weight: 400;
    display: flex;
}

.calculadora {
    position: absolute;
    background-color: rgb(250, 250, 250);
    top: 50%;
    left: 50%;
    transform: translate(-50%, -50%);
    border-radius: 15px;
    padding: 15px;
    box-shadow: 5px 4px 8px #88888821;
}

h1 {
    padding: 10px;
}

.botao {
    height: 50px;
    width: 50px;
    border-radius: 4px;
    font-size: 20px;
    cursor: pointer;
    background-color: #ffffff;
    color: #2b2b2b;
    border: none;
    margin: 3px;
    box-shadow: 1px 1px 2px #88888823;
}

.botaoi {
    height: 50px;
    width: 50px;
    border-radius: 4px;
    font-size: 20px;
    cursor: pointer;
    background-color: #515ce9;
    color: #ffffff;
    border: none;
    margin: 3px;
    box-shadow: 1px 1px 2px #88888823;
}

.botao:active {
    background-color: rgb(247, 247, 247);
    color: rgb(128, 128, 128);
}

.botaoi:active {
    background-color: #757ce2;
}

#resultado {
    background-color: rgb(250, 250, 250);
    color: black;
    font-size: 38px;
    width: 212px;
    height: 85px;
    margin: 5px;
    text-align: right;
    margin-top: 25px;
    padding: 3px;
}

.calculadora p {
    max-width: 15ch;
    overflow: hidden;
    text-overflow: ellipsis;
    white-space: nowrap;
}

footer {
    /*background-color: rgb(255, 255, 255);*/
    display: flex;
    text-align: center;
    justify-content: center;
    align-items: center;
    font-size: 10px;
    height: 50px;
    width: 100%;
    position: fixed;
    bottom: 0;
    left: 0;
}

footer a {
    text-decoration: none;
    color: inherit;
}

footer a:hover {
    color: brown;
}