Adding custom styles to Specific theme
Actually, what I would do is I would make my own sub-theme that uses Bartik as a base theme and add the additional css stylesheets to my sub-theme's .info file like this:

stylesheets[all][] = css/MY-CUSTOM-CSS-STYLESHEET.css


Just look at Bartik's own .info file to see how it adds stylesheets and javascript files as an example!

Here is some info on how to create a sub theme: http://drupal.org/node/225125.

The best thing about creating a sub-theme rather than adding these stylesheets using a custom module is that you'll be all set to do things like adding or overriding theme images, modifying template files and tackling a big array of potential customizations you may want to do in the future that you may not even be for-seeing now!  It'll just end up being more flexible in the long run.