discourse/app/assets/javascripts/admin/addon/adapters/theme.js

import RestAdapter from "discourse/adapters/rest";

export default class Theme extends RestAdapter {
  jsonMode = true;

  basePath() {
    return "/admin/";
  }

  pathFor(store, type, findArgs) {
    if (findArgs?.useConfigAreaEndpoint) {
      return "/admin/config/customize/themes";
    }
    return super.pathFor(...arguments);
  }

  afterFindAll(results) {
    let map = {};

    results.forEach((theme) => {
      map[theme.id] = theme;
    });

    results.forEach((theme) => {
      let mapped = theme.get("child_themes") || [];
      mapped = mapped.map((t) => map[t.id]);
      theme.set("childThemes", mapped);

      let mappedParents = theme.get("parent_themes") || [];
      mappedParents = mappedParents.map((t) => map[t.id]);
      theme.set("parentThemes", mappedParents);
    });

    return results;
  }
}