* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
}

#container {
    display: flex;
    justify-content: space-around;
    margin-top: 30px;
    padding: 5%;
    background: silver;
}
.caixa {
    background: black;
    color: wheat;
    padding: 15%;
    border-radius: 10px;
    text-align: center;
}