sS hCenterOverlay(O contents, O... _) {
  ret hcss([[.centerOverlay {
    position:absolute;
    top:50%;
    left:50%;
    transform:translate(-50%, -50%);
    color: white; background: #666666; opacity: .8;
    z-index: 1000;
  }]])
    + div(contents, params_plusHTMLClass(_, "centerOverlay"));
}