From Shopp Documentation

There are three levels of styles that are applied when using Shopp and they are ordered from least to greatest importance. The higher importance means the more style override capability you have:

• catalog.css
  A dynamic CSS file that provides core Shopp-specific element layout only
• shopp.css
  Shopp's template stylesheet providing the middleware styling and layout specific to your theme's needs
• styles.css
  Your active WordPress theme's stylesheet that should be able to override anything in catalog.css or shopp.css

The catalog.css file exists to provide the core layout for catalog elements when no other styles are present. It is loaded first and is therefore easily override-able by the later stylesheets - shopp.css and your theme's styles.css. Shopp styles can be placed in either shopp.css or your theme's styles.css file. Where you add them is a matter of preference.

You should never make a change to catalog.css as this file will be overwritten by any updates you make to Shopp in the future.