The various style sheets within the Drupal distro provide all the style definitions needed to format the site, and the various modules contained in the core. The individual theme you employ may also include additional styles that are particular to that specific theme.

Drupal contains a large number of style sheets—around twenty at the last count! While twenty is certainly a large number of style sheets to juggle, with good planning and use of overrides you can avoid the need to have to track down and modify individual style sheets—remember, we always want to avoid modifying the core files, and that includes the core CSS files.

Drupal is designed to deal with the complexity of this multi-layered approach to CSS gracefully. You can even configure the system to compile the various style sheets at run time, into one coherent list of styles—an option that not only eliminates any potential redundancies but also improves the performance of the site.

The order in which the...