Just a quick answer now, will give you some more info if you have more questions.
When you view a calendar URL, from your browser do a view page source. In the HTML Source you should be able to find the link for the url to the css file.
Copy that link from the http://....... to the end, and paste it in your browser address bar.
Your browser should open it or ask you to save it. YOu can save it locally using a name like "mycss.css".
Now you need to upload mycss.css TO YOUR SERVER! Everytime you make some change in mycss.css upload it again.
Now, in In jAC, on the CUSTOM CSS TAB near the bottom you can see a filed where you can enter the FULL URL to your server and CSS filename. For example:
http://www.yourserver.com/mycss.cssThe calendars should pick that up and use it whenever it displays your calendar.
John