.start-overlay{
  position:fixed;
  inset:0;
  display:flex;              /* hidden until script activates */
  align-items:center;
  justify-content:center;
  background:rgba(8,12,18,.72);
  backdrop-filter:blur(8px);
  z-index:99999;
}

/* visible state */
.start-overlay.hidden{
  display:none;
}

/* button */
.start-btn{
  min-width:190px;
  height:60px;
  padding:0 26px;
  border:0;
  border-radius:18px;
  cursor:pointer;

  font-size:20px;
  font-weight:800;
  letter-spacing:.3px;

  color:#081018;
  background:#5eead4;

  box-shadow:
    0 25px 60px rgba(0,0,0,.35),
    0 0 0 1px rgba(255,255,255,.08);

  transition:.25s ease;
}

.start-btn:hover{
  transform:translateY(-3px) scale(1.02);
}

.start-overlay.hide{
  opacity:0;
  transition:.28s ease;
}