hmm I have put the site in Default Mode (deactivated Root Folder BulletProof Mode), deleted the wp-admin htaccess file and the Plugin Firewall htaccess file is deleted and the CSS problem is still occurring. At this point it looks like this problem is not being caused by BPS Pro. Is there another .htaccess file higher up in the folder structure for this hosting account? Example: an htaccess file in the hosting account root folder could be affecting this website. If that is not the problem then what you want to try next is this: Use FTP or your web host control panel file manager and rename the /plugins/ folder to /_plugins/. It is possible that another plugin that you have installed is causing this problem.
Another place to check for another custom .htaccess file would be in the: /wp-content/.htaccess folder. If you see an .htaccess file there then rename it to: _.htaccess. BPS does not create an .htaccess file in the /wp-content/ folder, but other security plugins do do that.