php-flarum/js/admin/src/components/EditCustomCssModal.js

import SettingsModal from 'flarum/components/SettingsModal';

export default class EditCustomCssModal extends SettingsModal {
  className() {
    return 'EditCustomCssModal Modal--large';
  }

  title() {
    return app.translator.trans('core.admin.edit_css.title');
  }

  form() {
    return [
      <p>{app.translator.trans('core.admin.edit_css.customize_text', {a: <a href="https://github.com/flarum/core/tree/master/less" target="_blank"/>})}</p>,
      <div className="Form-group">
        <textarea className="FormControl" rows="30" bidi={this.setting('custom_less')}/>
      </div>
    ];
  }

  onsaved() {
    window.location.reload();
  }
}