THere's no generic
documentation because there's not defacto standard on how to package a theme, and how they're packaged makes some what of a difference in how to install them. Themes are usually packaged up with one of the following path structures:
1- /html/themes/<themename>
2- /themes/<themename>
3- /<themename>
In all cases, you upload <themename> to your /themes folder in your
PostNuke install. Sometimes you may find it a bit harder, just upload the entire folder (start at the folder level) that contains theme.php</themename></themename></themename></themename>