Hi
I dont think there is a propper documentation on theme modification anywhere. There is only this short doc on the specification i know of:
http://community.wdc.com/t5/WD-TV-Live-Hub-Themes/WDTVLiveHub-ThemeDefinition-v1-0-Here/td-p/373315
I dont have a need for a big theme change on this mediaplayer, but I made a small modification for the Mochi theme, and it was a lot of try end error, because you always have to see what number/variable does what…
Download a Theme, change some numbers and see what happens. The way is the goal. =)
If you get the hang of it, its very easy to do more/advanced modifications.