Your stylesheet is not showing because there is likely a permissions problem there caused by the .htaccess settings.
Zen Cart supplies some .htaccess settings in the /includes/.htaccess file which are intended to protect from inappropriate snooping of your files by hackers etc.
It seems your host doesn't like letting you control things that way, and thus you will have to rename or delete that file in order for your stylesheet to be accessible. Then you will need to get your hosting company to help you find another way to prevent unauthorized access to your .php files via a browser.
That may also help with your define pages being reachable; however, they are not accessed by the browser directly and thus the .htaccess should have no effect on them.




