html, body {
    height: 100%;
    margin: 0;
    padding: 0;
    display: flex;
    flex-direction: column;
    background-color: black;
    font-family: Verdana, Geneva, Tahoma, sans-serif;
    margin-top: 20px;
    font-weight: bold;
    color: green;
    text-align: center;
}
.content {
    flex: 1;
    padding: 75px 20px;
}
* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
}
h1 {
    text-align: center;
    color: blue;
}
ul {
    list-style-type: none;
    padding: 0;
    max-width: 400px;
    margin: auto;
    color: white;
}
li {
    margin-bottom:  15px;
}
laber {
    display: block;
    margin-bottom: 5px;
    font-weight: bold;
}
input[type="number"]{
    width: 100%;
    padding: 8px;
    box-sizing: border-box;
}
button {
    padding: 10px 20px;
    font-size: 16px;
    cursor: pointer;
}
.content{
    flex: 1;
}
#mansagem {
    margin-top: 20px;
    font-weight: bold;
    color: green;
    text-align: center;
}
footer {
    background-color: #333;
    color: wheat;
    text-align: center;
    padding: 15px;
    margin-top: 0%;
}