#myVideo {
  z-index: -100;
  position: fixed;
  right: 0;
  bottom: 0;
  min-width: 100%;
  min-height: 100%;
  display: block;
  filter: opacity(0.5);
}

<!-- https://css-tricks.com/almanac/properties/f/filter/ -->

#myBtn {
  outline: none;
  border: none;
  width: inherit;
  height: inherit;
  background-color: inherit;
  cursor: pointer;
  color:  LightGray;
  padding: 0px 6px;
  align-items: center;
  text-align: right;
}

#myBtn:hover {
  // background: rgba(211, 211, 211, 0.2);
  color: white;
  font-size: 12pt;
}