body {
  padding-top: 50px; /* adjust to your .top-bar height */
}

.top-bar > .row {
  position: fixed;
  top: 0;
  left: 0;
  right: 0;
  z-index: 1030; /* Bootstrap navbar uses 1030, so this is good */
  background-color: #212529; /* same as bg-dark */
}