img.obrazek:hover {
  border-color: #0066cc;
}

.overlay {
  position: fixed;
  top: 0; left: 0; right: 0; bottom: 0;
  background: rgba(0,0,0,0.9);
  display: flex;
  align-items: center;
  justify-content: center;
  z-index: 1000;
}

.overlay.hidden { display: none; }

.overlay img {
  max-width: 90%;
  max-height: 80%;
}

.nav {
  position: absolute;
  top: 50%;
  font-size: 3rem;
  color: #fff;
  cursor: pointer;
  user-select: none;
  transform: translateY(-50%);
  padding: 10px;
}

#prev { left: 20px; }
#next { right: 20px; }

.close {
  position: absolute;
  top: 10px; right: 20px;
  font-size: 2rem;
  color: #fff;
  cursor: pointer;
}