|Developer Documentation Overview / Developing Custom Themes|
The theme resource kit is a set of sample files that make up a theme, arranged in the hierarchy that the application expects. You do not have to use this kit, but it can be helpful to understand the kinds of things you can modify in the application.
To make the changes described here, you'll need administrator privileges and a good understanding of Freemarker or Soy templates.
The directory hierarchy in your theme is important; things will break if you don't follow the correct pattern. What's in the resource kit is a good example, with a root directory that has subdirectories for images, styles, and template. Within the template directory, the hierarchy is also important.