When creating and deploying a cutom theme, each theme needs its own .inf file and will require a reference in the SPTHEMES.XML file in this format.
Usually, this can be found at <drive>\Program Files\Common Files\Microsoft Shared\web server extensions\12\TEMPLATE\LAYOUTS\1033\SPTHEMES.XML.
Add the following lines under <SPThemes> tag:
Notice that preview and thumbnail paths are images/thghost.gif. By default, MOSS 2007 and WSS 3.0 will not have such image files. These will also need to be manually added to the images folder.
WARNING: Whenever deploying a change to an existing theme, after deploying the change and performing an iisreset on the desired server, the site theme has to be reapplied through Site Actions > Site Admin on EACH affected Site Collection.