Thanks for the reply.

I actually already have my own theme (THMX) file. What I want is for my add-in macro to apply this theme. My code would be:

ActiveWorkbook.ApplyTheme ("C:\Program Files (x86)\Microsoft Office\Document Themes 14\Custom.thmx")
I was hoping the THMX (or XML) file could somehow be included in the add-in (because users won't have it), but maybe this just is not possible.