Redmine - Help: RE: My issues do not appear following a migration

I think the theme you've installed inherits the original theme. I would try to reinstall the basic theme by downloading the appropriate application.css from the repository and copying it over to public/stylesheets/. I would also check permissions of these files and folders. If it's a missing file from the server one could open the browser console and check if all files are successfully transmitted by the server. There shouldn't be any 404 response.