I had the same problem, with the setup.py installation to a desktop running Debian 11 and uploading once a minute via ftp to a remote web server.
I compared the files on localhost/weewx/ with those on the remote server and found the server missing several files, including seasons.css, seasons.js, and two .woff2 files in a font directory. I stopped weewx, ftp-ed the missing files, and restarted. Now when I use a browser to view the page on the remote server, everything looks as it should.