.bola {
  position: absolute;
  width: 1080px;
  height: 1080px;
  background: radial-gradient(circle, rgba(0,0,255,0.6) 0%, rgba(0,0,255,0) 70%);
  border-radius: 50%;
  top: 100px;
  left: 50%;
  transform: translateX(-50%);
  z-index: -1;  /* fica atrás do conteúdo */
}