.ui-dialog-titlebar-close {

  background: white
  
}

.ui-dialog-titlebar-close::before {
  content: "X";
  position: absolute;
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%);
  font-size: 2rem;
  line-height: 1;
  font-weight:bold;
  color:#ffffff;
}