Upgrade via web interface = CSS error

I was doing a routine upgrade using the web interface and everything was going fine - until I accidentally left the upgrader “maintenance mode” instead of clicking the option (yes/no - I don’t remember which was which) you use for upgrading via the web interface rather than the CLI.
Everything is running fine, except what looks to be the CSS on the web interface after you sign in.
Can this be fixed without messing with the user accounts, data, so on so forth? I’m not sure where in the /var/www/html/nextcloud directory (running on my own server - Ubuntu 18.04 LTS) to move/copy/reinstall an old or existing CSS file from.
Also I’m not certain that just copying a CSS file would fix this. Is there any way to reinstall the current upgrade from the CLI? (I’m sure there is but I haven’t found it yet - if someone could point me to that it would be much appreciated!)