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 Method for running themes.
1. Upload theme via WD Official Webend
Limitations and Requirements of Official Method
1. Theme must be inside a .zip file
2. Size can not exceed 30MB
3. zipped theme just not use any compression
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. .