to match the underlying call to SetId in GalleryTheme

Change-Id: If862c1f7c2d7c05484347ef46a9d504e95686ec7
Reviewed-on: https://gerrit.libreoffice.org/46273Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>