#gray-mode-toggle {
  position: fixed;
  bottom: 20px;
  right: 20px;
  background: #444;
  color: white;
  border: none;
  border-radius: 50%;
  width: 50px;
  height: 50px;
  font-size: 20px;
  cursor: pointer;
  box-shadow: 0 0 10px rgba(0,0,0,0.3);
  z-index: 9999;
  transition: background 0.3s;
}
#gray-mode-toggle:hover {
  background: #666;
}

body.gray-mode {
  filter: grayscale(100%) brightness(105%) contrast(110%);
  transition: filter 0.5s;
}