Hi,

I've digged on teme changing trouble and I've got a response.
If you have a theme already installed (before you install maenotify), switching to this should not giving you any error.
If you have installed maenotify and after you installed a new theme the switching to this new one will be a problem.
In particular this is a permission error, to fix it, the current workaround is to re-install maenotify.