Tiki is an application designed to grow in a neat and usable way, there are several features that allow this application to be easily enhanced and customized by programmers.
Smarty (the template engine) is used to sepparate content from presentation, all the logic is in the foo.php files and there s a foo.tpl file that contains the template used to display what the php file generates.
The whole layout of the application is present in only one file: tiki.tpl (the main template) so you can change the layout as you want by changing just this file
CSS is used to define the presentation properties of all the elements in the Tiki pages, themes are defined creating new CSS files and putting them in the CSS directory.
All the database-accessing functions are in library files in the lib directory as classes, the code in PHP files access/changes entities by calling the Tiki API.If you code new pages/modules/whatever please respect this, thanks!
Tiki is really easy to customize/extend, please give it a chance!