body {background-image: url("https://stromypronamesti.cz/grafika/stromy-pozad.jpg"); background-repeat: no-repeat; background-size:cover; background-attachment: fixed}
.centered{
position: absolute;
  width: 400px;
  height: 500px;
  z-index: 15;
  top: 50%;
  left: 50%;
  margin: -250px 0 0 -200px;
  color: black;
  text-shadow: 2px 2px 7px black;
  text-align: center;
}
.navi a{color: black; text-shadow: 2px 2px 7px black;}


img.grayscale {
  filter: url("data:image/svg+xml;utf8,<svg xmlns=\'http://www.w3.org/2000/svg\'><filter id=\'grayscale\'><feColorMatrix type=\'matrix\' values=\'0.3333 0.3333 0.3333 0 0 0.3333 0.3333 0.3333 0 0 0.3333 0.3333 0.3333 0 0 0 0 0 1 0\'/></filter></svg>#grayscale"); /* Firefox 3.5+ */
  filter: gray; /* IE6-9 */
  -webkit-filter: grayscale(100%); /* Chrome 19+ & Safari 6+ */
}

img.grayscale:hover {
  filter: none;
  -webkit-filter: grayscale(0%);
}