body {
  margin: 0;
  padding: 0;
  background: url(https://occ-0-325-395.1.nflxso.net/dnm/api/v6/6AYY37jfdO6hpXcMjf9Yu5cnmO0/AAAABaKr-dQAdVTt7fuGCgzntgBBrFce2DMW72dF86eO7EnXbFZvzmX2TPnQAg3HwAsvt7ZnDnP0nwuHOtPwpWGGOE22fXq2.webp?r=847) top/contain no-repeat;
}

nav {
  position: -webkit-sticky;
  position: sticky;
  /* sticky or fixed are fine */
  position: fixed;
  top: 0;
  height: 69px;
  width: 100%;
  background: linear-gradient(to bottom, #000, #0003 70%,#0000); /* background when scroll is in the top */
  transition: background .5s; /* control how smooth the background changes */
}

nav.scrolled {
  background: #0a0a0a;
}

main {
  height: 200vh;
}