Disable for admin theme?

Created a patch as per the request, Please review.

I think it would be better/more flexible to create a setting named 'Enable for' with two checkboxes, 'Admin theme' and 'Frontend theme'. This way the module can also be used only in the admin theme. The two checkboxes should be enabled by default for both new and existing installations, to preserve backwards compatibility.

Or even add a checkbox to each individual theme's settings. Maybe that's a followup ;-)