Plone’s style sheets are managed by a tool in the ZMI called portal_css. It looks like this:
As mentioned, you should enable Debug/development mode during development. The Automatic grouping mode option can improve performance if you use a lot of custom style sheets, but introduces a small risk of style sheets being included in a different order to what the designer intended. Turn this on if it does not cause any problems for your theme.
The various style sheets listed will be collated (and possibly concatenated, if development mode is off) and rendered in the page sent to the web browser. The installer for the