Drupal is designed to allow for relatively easy changes to the look-and-feel of the website. PHP code dedicated to the user presentation is intended to be housed in special template files for site developers to customize. The Drupal package comes with several themes as part of the default offering, however thousands of user-contributed themes are available for site-developers to download, install, and almost instantly change the look and feel of their sites. A Drupal Theme repository is available for searching user-contributed themes.
For default themes that come with Drupal:
[drupal install path]/themes
For 3rd-party installed themes:
[drupal install path]/sites/all/themes