I've just been (struggling) to create a new theme for SharePoint but kept getting the following error:

A theme with the name "MY-THEME-HERE 1011" and version already exists on the server

It turns out that my ThemeID (and the name of the folder) were too long - how the error message above gives any indication of that is beyond be! Anyway all you need to do is make sure the ThemeID is a maximum of 8 characters. Once I shortened it everything worked fine.

I'm new to SharePoint so ignore me if this 'common knowledge' - I thought I'd share for any other newbies out there...