How to resolve the problem
Contact your web hosting provider and let them know you need the rule 340163 whitelisted for domain.com as its hitting a mod_sec rule
More details about this problem
If you are trying to update the background URL (Producer CSS Styling Tool) or the logo URL and you are getting the message You don’t have permission to access /wp-admin/themes.php on this server or “Page Does Not Exist”, this problem is coming from your server and not from the theme.
For those of you who are getting these messages after you’ve updated to a new version of the theme, it may appear to be resulting from updating to the new version, but it is actually that mod security changes were made to your server at some point since you last updated any of the theme fields that attempt to submit URL data. Because you are only now submitting URL data since updating the theme and since the security changes were mad to your server you are only now being affected by the mod security changes.
Other similar discussons on the web:
http://wordpress.org/support/topic/396292
