Custom TemplateRootPaths in backend mode filelist (upgrade v11 -> v12)

how can I change TemplateRootPaths in FileListController.php?

BackendViewFactory.php line 73:
// @todo: This assumes the pageId is always given as ‘id’ in request.
// @todo: It would be cool if a middleware adds final pageTS - already overlayed by userTS - as attribute to request, to use it here.
$pageTs = [];
$pageId = $request->getParsedBody()[‘id’] ?? $request->getQueryParams()[‘id’] ?? 0;
if (MathUtility::canBeInterpretedAsInteger($pageId)) {
// Some BE controllers misuse the ‘id’ argument for something else than the page-uid (especially filelist module).
// We check if ‘id’ is an integer here to skip pageTsConfig calculation if that is the case.
// @todo: Mid-term, misuses should vanish, making ‘id’ a Backend convention. Affected is
// at least ext:filelist, plus record linking modals that use ‘pid’.
$pageTs = BackendUtility::getPagesTSconfig((int)$pageId);
problem is here, because for filelist $pageId = ‘1:/’, so pageTsconfig not included.