just to join the party 
changing the theme/apparence is definetely allowed
as WD has even provided support and ways to change the theme running on your device
you may also notice there’s even an entire sub-forum WD has provided related to themes
and if you go to the appearance settings menu on the device, WD has even hosted a couple of theme’s for direct download from wd servers, and they include a few from forum users PsychoTHC, and Julios
so to speak as one that has done all the above
I’ve done theme’s, I’ve done custom firmware, I’ve modified GPL code, etc
my recommendation is to skip the GPL code entirely, it’s overkill for what you want to do
GPL code contains stuff like the linux kernel source code
and other binary files that you would need to create your own firmware
myself and joey have already provided all the xml’s and graphics needed
links can be found here http://community.wd.com/t5/WD-TV-Live-Streaming/Stock-OSD-Mochi-2-01-86/m-p/642525#M25989
also be sure to check out the sub forum
http://community.wd.com/t5/WD-TV-Live-Hub-Themes/bd-p/hub_themes
there’s a lot of great user designed themes available
