@media only screen and (max-width:800px) {
  /* For tablets: */
  #content-box {
    width:80%;
  }
  .#content-menu {
    width:100%;
  }
}
@media only screen and (max-width:500px) {
  /* For mobile phones: */
  .menu, .main, .right {
    width:100%;
  }
}