.header {
  display: block;
  margin-left: auto;
  margin-right: auto;
}

body {
    background-image: url("b5.gif");
    background-repeat: repeat;
    background-attachment: fixed;
    background-color: midnightblue; 
    color: white;
}

  
a {
    color: blueviolet;
}
  
a:visited {
    color: darkslateblue;
}