Drupal contains a large number of stylesheets—more than forty at the last count! If you are employing third-party themes or modules, you are also likely to encounter additional stylesheets that are particular to that specific extension.
While there are certainly a lot of stylesheets to juggle, with good planning and use of overrides you can avoid having to track down and modify individual stylesheets. Indeed, as you will be placing your new styles in the theme directory, you won't need to work much with the multitude of the system's default stylesheets.
Note
Remember, you always want to avoid modifying the default files, and that includes both the CSS files in the core and those in any additional installed modules.
Drupal deals gracefully with the complexity of its multilayered approach to CSS. The system includes a CSS compression option (see, Administer | Site configuration | Performance) that will automatically compile the various stylesheets into one coherent...