The Kodi wiki pages for skinning are the best available resource at this time -- combined with the default Plex skin.
This thread is also important -- shows the changes of the Kodi skinning engine from Frodo onwards (PHT currently uses Frodo, although it appears that may change any day now): http://forum.kodi.tv/showthread.php?tid=231270 Some of the things you find in the wiki pages will need to be changed to the earlier frodo equivalents (left needs to be posx, alight right starts from the right instead of the left, etc.)
The most significant differences between Kodi and Plex for skinning will the various IDs, and listitem properties. Those can be found through examining the default skin.
Also, it does appear a major overhaul of PHT is coming soon which according to the this thread (https://forums.plex.tv/discussion/172664/pht-development/p1) will break all existing skins.