Every Drupal site uses a theme (or two!). It can be a free core or contributed theme. You can buy one from a theme vendor, for example, topnotchthemes.com or themesnap.com. Or, you can get fancy and create your own! In this section, we'll look at some considerations for multilingual theming including RTL support and custom CSS.
Many developers create modules to handle custom site features. This section also briefly covers internationalization for custom modules, including using string functions for hard-coded and user-defined strings. To learn more about theme and module development, check out the Drupal 7 Themes book by Ric Shreves and the Drupal 7 Module Development book by Matt Butcher, Larry Garfield, John Wilkins, Matt Farina, Ken Rickard, and Greg Dunlap.