Hello,
I was thinking of the something similar: to create downloadable theme packages for this plugin (it has an individual theme structue inside wp).
I began make some little changes in the original theme using child theme. I think this is the only (secure) way to make themes. The only question is which files can be overriden using a child theme. I think every file of the parent theme, but I haven’t tried it yet.
I have tried to figure out how calendar themes working. There is a good article to start on time.ly (about calendar theme structure)
http://time.ly/documentation/reference-calendar-theme-structure/. It uses variables.php with the basic settings, I think it can be created from scratch. There are css less files with built in variables and values, these can be modified, too.
What I miss is the “big picture” how this files work together and how css rendering system works. And the top of all parts of the plugin based on twitter bootstrap, an other piece of code set.
I have some basic tips how I have tried to change style (very basic changes) at wpfiles dot wordpressdotcom
