Drupal: Overriding CSS stylesheets
Ah, das hatte ich schon mal irgendwo gelesen, aber eine gute Erinnerung.
Take the link module, which adds a link field type to CCK. This module has provides a CSS file : link.css. Suppose you have uploaded your module at sites/all/modules/contribute/link, we now have a file sites/all/modules/contribute/link/link.css.
To replace this file with our own CSS style in our theme, we just copy this file to our theme. So if our theme is at sites/all/themes/my_theme, the CSS file is at sites/all/themes/my_theme/link.css. We can now put our own styles in there.