Unlike the WDTV Live hub, The SMP uses the flash memory to store the user Themes and there is only enough room for one user supplied theme, That theme needs to be slim and not take too much of flash space.
The WDTV Live SMP will refuse to accept the user theme if it exceeds the file size, It will also refuse if the directory structure is not correct
The correct way to Zip a file would be like this…
This needs to be zipped and a filename given
If WDTV SMP likes the file structure/Size it will display congratulations! from the Web UI
See this post, user shadow_cruiser has already documented the steps.