body,html{
    height: 100%;
    margin: 0;
}

.bg{
    background-image: url("fundo_colonia.jpg");
    background-repeat: no-repeat;
    background-size: cover;
    background-position: center;
    background-attachment: fixed;
  }

#centro {
    width: 100%;
    height: 50px;
    line-height: 50px;
    text-align:center;
    background-color: rgba(10,23,55,0.7);
    color: white;
    
    /* pura mágica */
    position: absolute;
    top: 50%; /* posiciona na metade da tela */
    margin-top: -25px; /* e retrocede metade da altura */
  }