IMPORTANT: In Drupal 6, the theme system caches template files and which theme functions should be called. What that means is if you add a new theme or preprocess function to your template.php file or add a new template (.tpl.php) file to your sub-theme, you will need to rebuild the "theme registry." See the Theming Guide

Drupal 6 also stores a cache of the data in .info files. If you modify any lines in your sub-theme's .info file, you MUST refresh Drupal 6's cache by simply visiting the admin/build/themes page.

The base Zen theme is designed to be easily extended by its sub-themes. You shouldn't modify any of the CSS or PHP files in the zen/ folder; but instead you should create a sub-theme of zen which is located in a folder outside of the root zen/ folder. The examples below assume zen and your sub-theme will be installed in sites/all/themes/, but any valid theme directory is acceptable (read the sites/default/default.settings.php for more info.)

Why? To learn why you shouldn't modify any of the files in the zen/ folder, see Why use Zen?

1. Copy the STARTERKIT folder out of the zen/ folder and rename it to be your new sub-theme. IMPORTANT: Only lowercase letters and underscores should be used for the name of your sub-theme.

   For example, copy the sites/all/themes/zen/STARTERKIT folder and rename it as sites/all/themes/foo.

   Why? Each theme should reside in its own folder. Unlike in Drupal 5, sub-themes can (and should) reside in a folder separate from their base theme.

2. In your new sub-theme folder, rename the STARTERKIT.info.txt file to include the name of your new sub-theme and remove the ".txt" extension. Then edit the .info file by changing any occurrence of STARTERKIT with the name of your sub-theme and editing the name and description field.

   For example, rename the foo/STARTERKIT.info.txt file to foo/foo.info. Edit the foo.info file and change "STARTERKIT.css" to "foo.css", change "name = Zen Sub-theme Starter Kit" to "name = Foo", and change "description = Read..." to "description = A Zen sub-theme".

   Why? The .info file describes the basic things about your theme: its name, description, features, template regions, CSS files, and JavaScript files. See the Drupal 6 Theme Guide for more info.

   Then, visit your site's admin/build/themes to refresh Drupal 6's cache of .info file data.

3. If you want a liquid layout for your theme, copy the layout-liquid.css from the zen/zen folder and place it in your sub-theme's folder. If you want a fixed-width layout for your theme, copy the layout-fixed.css from the zen/zen folder and place it in your sub-theme's folder. Rename the layout stylesheet to "layout.css".

   For example, copy zen/zen/layout-fixed.css and rename it as foo/layout.css. Note that the .info file already has an entry for your layout.css file.

   Why? In Drupal 6 theming, if you want to modify a stylesheet included by the base theme or by a module, you should copy the stylesheet from the base theme or module's directory to your sub-theme's directory and then add the stylesheet to your .info file. See the Drupal 6 Theme Guide for more info.

4. Copy the zen stylesheet from the zen folder and place it in your sub-theme's folder. Rename it to be the name of your sub-theme.

   For example, copy zen/zen/zen.css and rename it as foo/foo.css. Note that the .info file already has an entry for your foo.css file and that your .info file removes the base theme's zen.css file.

5. Copy the print stylesheet from the zen folder and place it in your sub-theme's folder.

   For example, copy zen/zen/print.css to foo/print.css. Note that the .info file already has an entry for your print.css file.

6. Copy the html-elements stylesheet from the zen folder and place it in your sub-theme's folder.

   For example, copy zen/zen/html-elements.css to foo/html-elements.css. Note that the .info file already has an entry for your html-elements.css file.

7. Edit the template.php and theme-settings.php files in your sub-theme's folder; replace ALL occurrences of "STARTERKIT" with the name of your sub-theme.

   For example, edit foo/template.php and foo/theme-settings.php and replace "STARTERKIT" with "foo".

8. Log in as an administrator on your Drupal site and go to Administer > Site building > Themes (admin/build/themes) and enable your new sub-theme.
   

   Optional:

9. Modifying Zen core stylesheets: If you decide you want to modify any of the other stylesheets in the zen folder, copy them to your sub-theme's folder before making any changes. Also, be sure the new stylesheet is included in your .info file and that you have rebuilt the theme registry.

   For example, copy zen/zen/block-editing.css to foo/block-editing.css. Then edit foo/foo.info and uncomment this line to activate it:
   ;stylesheets[all][] = block-editing.css
   to:
   stylesheets[all][] = block-editing.css
   Then, visit your site's admin/build/themes to refresh Drupal 6's cache of .info file data.

10. Modifying Zen core template files: If you decide you want to modify any of the .tpl.php template files in the zen folder, copy them to your sub-theme's folder before making any changes. And then rebuild the theme registry.

    For example, copy zen/zen/page.tpl.php to foo/page.tpl.php.

11. Themeing Drupal’s search form: Copy the search-theme-form.tpl.php template file from the modules/search/ folder and place it in your sub-theme's folder. And then rebuild the theme registry.
    

    Why? In Drupal 6 theming, if you want to modify a template included by a module, you should copy the template file from the module's directory to your sub-theme's directory and then rebuild the theme registry. See the Drupal 6 Theme Guide for more info.

12. Further extensions of your sub-theme: Discover further ways to extend your sub-theme by reading Drupal 6's Theme Guide online.