[Koha] Problem with upgrading Koha from 17.11 to 18.11

[Daniel Owens]

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