Gen3 aka (WD Streaming Media Player or SMP) has official supported methods for themes as well as WDLXTV theming methods.
All themes once loaded can be selected in the settings menu. This is true of both official and WDLXTV theming methods.
Official Methods for running themes.
1. Upload theme via WD Official Webend
2. Select a Downloadable theme from WD servers (4 or 5 available)
Limitations and Requirements of Official Method
1. Theme must be inside a .zip file
2. Size can not exceed 30MB
3. zipped theme must not use any compression
4. Can only have 1 theme at a time. (uploading/downloading a different theme erases the one previously installed)
5. Must have a valid meta.xml (this is actually true of any theme, regardless of method used)
WDLXTV additional Methods for running themes.
Any of the following are valid folder names for running a theme.
Theme folder can be located on any attached drive.
This is an bit depreciated now, but can still be used to force the firmware to boot to this theme at each boot.
General Theme with added benefit of automated linksheets scanning via linksheets.pl script
Which method is best for me?
I recommend you read the Gen3 Moviesheets wiki and then decide.
Where can I find additional themes?
http://forum.wdlxtv.com/viewforum.php?f=13 (very few gen3 themes can be found here)
http://community.wd.com/t5/WD-TV-Themes/bd-p/hub_themes (most gen3 themes will be found here)
I want to create my own theme. How do I start?
Start by Downloading the stock theme. http://www.mediafire.com/download/tkmbp6zg31b3h1i/Grid-UI.2.02.32.zip
Using WDLXTV methods, you can have as many themes as you want on your thumbdrive
WDTV Hub, WDTV Gen3, WDTV Gen4 themes are for the most part compatible, with a few minor glitches