Apple Mac OS X Server (Extending Your Wiki Server For Version 10.5 Leopard) - Manual (Page 8)

However, CSS files located in individual theme folders take precedence and override
the settings in this folder.
Individual theme folders are subfolders with a .wikitheme extension that are located
within this folder:
/Library/Application Support/Apple/WikiServer/Themes/
/Library/Application Support/Apple/WikiServer/Themes/
compressed/
theme_name/
screen.css
/usr/share/collaboration/themes/
theme_name/
screen_compressed.css
Imports
If the theme has a /compressed subfolder, it imports the CSS files located at:
/usr/share/collaboration/themes/theme_name/screen_compressed.css
The screen_compressed.css file combines the many individual CSS files located in the
theme_name/screen/ folder. These are provided in the prebuilt themes to improve
overall performance. The screen_compressed.css file isn’t created automatically or
dynamically, so any changes to .css files located in the theme_name/screen/ folder
don’t affect the screen_compressed.css file.
8 Chapter 1 Customizing How the Wiki Looks