Sorry, I do not specialist on questions creating sinelerra's themes...
But I would be interested to read a guide on how to do it.
There might have been a guide available in the CV version in the past, but I have not located one. It might be worthwhile to add one to the appendix in the CinelerraGG_manual.pdf in the future.
Meanwhile if you look at {cinelerra_path}/plugins/theme_cakewalk directory you can see what is involved. For comparison, in the plugins subdirectory each of the themes has its own subdirectory, named theme_... (for example theme_suv or theme_hulk). If you compare these, you can see that they all have similar formats with the majority of original work done in png and svg files.