SharePoint's features are installed into the 12 hive folder
C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\12\TEMPLATE
Features like themes require a change in SPTHEMES.XML located at
C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\12\TEMPLATE\1033
However, if your MOSS installation comes in other language rather than english, this folder will differ on the Local ID (LCID).
For example, portuguese (ID 2070):
C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\12\TEMPLATE\2070
This means you have to check the web server's language ID BEFORE make any changes.
Language IDs determine the languages used for text in the Web pages, such as text on the Site Settings page. Languages available for site creation depend on the language template packs that you have installed on the server or server farm.
Locale IDs control the numbering, sorting, calendar, and date and time formatting for the Web site. You can change the locale for a Web site by using the Regional Settings page.