discourse/app/assets/javascripts/admin/addon/controllers/modals/admin-theme-change.js

import Controller from "@ember/controller";
import ModalFunctionality from "discourse/mixins/modal-functionality";
import { ajax } from "discourse/lib/ajax";

export default Controller.extend(ModalFunctionality, {
  loadDiff() {
    this.set("loading", true);
    ajax(
      "/admin/logs/staff_action_logs/" + this.get("model.id") + "/diff"
    ).then((diff) => {
      this.set("loading", false);
      this.set("diff", diff.side_by_side);
    });
  },
});