Site Specific CSS

For Drupal 6, consider using Custom CSS and JavaScript files

In a multisite environment, sites may want to use the same theme, which resides under sites/all/themes.
It is quite often that users wish to personalize the theme with new CSS rules.
This module will look for another CSS file, located under the site's file directory, and named same as the theme.

For example, to override Garland's CSS rules, only for a specific site, without duplicating the theme itself, just create another CSS file named garland.css under the sites files directory (e.g., sites/example.com/files/garland.css)

This module is specially good when you want to allow users some design freedom, without letting them install their own themes.