Where are CSS files in Drupal

Cascading style sheets (CSS) are a standard technology for controlling the presentation of web pages. When a page is called by a browser, one or more .css files provide information about how the content on the page should be displayed. The specifics of CSS are beyond the scope of this guide. For a general overview, see the Cascading Style Sheet article on Wikipedia.

A CSS file in a Drupal theme is no different than any CSS file on the web. However, the procedure for adding a CSS file to a theme is unique to Drupal. Drupal 5 (and earlier) and Drupal 6 (and later) have different procedures for adding a CSS file. Consult the version-specific guides for more information.

Some modules (both core and contributed) provide their own default CSS files for controlling the presentation of the content they generate. While you can theoretically edit these files directly, it is highly recommended that you "override" a default .css file by copying (not moving) it to your theme's directory.