[Koha] Problem with upgrading Koha from 17.11 to 18.11

I just upgraded from 17.11 to 18.11. Afterward, both the staff client and OPAC display very basic HTML, with no styling. The staff client login looks like this: Koha <http://koha-community.org/> Username: Password: Library: * English * Tiếng Việt <http://thuvien.hbc.edu.vn:8080/cgi-bin/koha/changelanguage.pl?language=vi-VN> I can log in to the staff client, and I can run searches in OPAC, but the pages are not displaying correctly. It is almost as if stylesheets were not included in the upgrade. Can anyone suggest how I can troubleshoot this problem? Daniel