Enabling Clean URLs on website-in-progress
You can use relative css as long as you are pointing to files within your theme. That will work like this. url('img/image.jpg'); So no need for full url's here.

More explanation here: http://css-tricks.com/quick-reminder-about-file-paths/

At template level you might want to use drupal_get_path to point to the various modules and or themes.