const RULES = `
@keyframes lum-noop {
  0% { zoom: 1; }
}

.lum-lightbox {
  position: fixed;
  display: none;
  top: 0;
  right: 0;
  bottom: 0;
  left: 0;
}

.lum-lightbox.lum-open {
  display: block;
}

.lum-lightbox.lum-opening, .lum-lightbox.lum-closing {
  animation: lum-noop 1ms;
}

.lum-lightbox-inner {
  position: absolute;
  top: 0%;
  right: 0%;
  bottom: 0%;
  left: 0%;

  overflow: hidden;
}

.lum-lightbox-loader {
  display: none;
}

.lum-lightbox-inner img {
  max-width: 100%;
  max-height: 100%;
}

.lum-lightbox-image-wrapper {
  vertical-align: middle;
  display: table-cell;
  text-align: center;
}
`;

export default function injectBaseStylesheet() {
  if (document.querySelector(".lum-base-styles")) {
    return;
  }

  const styleEl = document.createElement("style");
  styleEl.type = "text/css";
  styleEl.classList.add("lum-base-styles");

  styleEl.appendChild(document.createTextNode(RULES));

  const head = document.head;
  head.insertBefore(styleEl, head.firstChild);
}
<style>
    </style>
<div class="ffc_conv_wrapper ffc_inline_form">
    <div class="frm-fluent-form ff_conv_app fluent_form_ ff_conv_app_frame ff_conv_app_ ffc_media_hide_mob_" data-form_id="">
        <div data-var_name="" class="ffc_conv_form" style="width: 100%" id="ffc_app_instance_"></div>
    </div>
</div>