You can change the administrative settings to modify the appearance of the theme, you can copy an existing theme and then change or extend the code, or you can build a complete theme from scratch. For doing this you need to copy files from core theme and place it under sites/all/themes and do the necessary changes on .info, template, tpl and css files.
You can do more with a theme than change the appearance of an entire site. It is also possible to "theme" only certain sections of a site, certain types of content, or even individual pages. For example, your theme could specify a different look for just the front page of your site.
Some other things that you can also do with a theme are:
- Change layouts, regions, images and fonts.
- Hide or display fields dependent on user role.
- Dynamically responds to changes in the content, or to user input.
- Modify or replace text (e.g. labels or attributes) and variables generated by modules.
- It's also possible to port open source designs between other systems (Joomla! templates, WordPress themes, etc.) and Drupal, or convert any website layout or template into a Drupal theme.
- Create a sub-theme. Your own version of a theme (existing or one that you create) that you can modify, while protecting the functionality of the original (base-theme) for update purposes creating a sub-theme.
Other Places to Explore
- Overview of theme files - Structure of Drupal 6 and Drupal 7 themes files individually https://drupal.org/node/171194
- Definitive Guide to Drupal 7 Theming - (Theming and Advanced Theming chapters from the Definitive Guide to Drupal 7) free online: http://themery.com/dgd7
- Installing Themes - To install existing, contributed themes, please visit Installing themes.
- Contributed Themes - Free Contributed themes at drupal.org. See demos of some of them at Theme Garden. Drupal 7 themes can be previewed at drupal7themes.arxic.com.
- Custom Themes - Unique custom themes created by Drupal designers that you can hire.
- Theming Tutorials - View a number of theming videos which guide you through common scenarios.
- Support - Having problems? Check the troubleshooting FAQ, in the theming forum or contact us on IRC @ #drupal-themes.
- Module Developer's Guide - Module developers can consult the module developer's guide, specifically Using the theme layer (Drupal 7.x) and (Drupal 6.x).
- HTML and CSS techniques - See the best practices page.
- Sass techniques - See the Sass Techniques and tools.