How TO - Change Background on Scroll

body, html {
  height: 100%;
  margin: 0;
  font-family: Arial, Helvetica, sans-serif;
}

* {
  box-sizing: border-box;
}

.bg-image {
  /* Full height */
  height: 50%;

  /* Center and scale the image nicely */
  background-position: center;
  background-repeat: no-repeat;
  background-size: cover;
}

/* Images used */
.img1 { background-image: url("img_snow.jpg"); }
.img2 { background-image: url("img_girl.jpg"); }
.img3 { background-image: url("img_lights.jpg"); }
.img4 { background-image: url("img_nature.jpg"); }
.img5 { background-image: url("img_forest.jpg"); }
.img6 { background-image: url("img_woods.jpg"); }

/* Position text in the middle of the page/image */
.bg-text {
  background-color: rgb(0,0,0); /* Fallback color */
  background-color: rgba(0,0,0, 0.4); /* Black w/opacity/see-through */
  color: white;
  font-weight: bold;
  font-size: 80px;
  border: 10px solid #f1f1f1;
  position: fixed; /* Stay fixed */
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%);
  z-index: 2;
  width: 300px;
  padding: 20px;
  text-align: center;
}