Why might Growl's theme or settings not work?
Symptoms & Resolution
If you've Growl 1.3 or later installed on your Mac, you'll be able to customize how notifications put out by Due look and behave.
If Due is not respecting the theme or settings as set in Growl, ensure that you're using Growl 1.3 or later by launching Growl, then choosing About.
Notifications continue to work whether or not you have Growl 1.3 installed.
However, to customize how notifications look and behave for Due, you'll need to have at least Growl 1.3 or later. This is because Due is fully sandboxed per Apple's App Store guidelines, and can only communicate with the new version of Growl.
By June 2012, Apple requires all apps on the App Store to be sandbox compliant as well. Eventually all apps on the App Store that support Growl would have to move to the new version of Growl too.