Thanks Todd,
1) No there isn't. The best solution I can think of is to use an image, but that isn't ideal if all you want is to change the colour. The best solution would be to create another 'background' element that you can choose to add to the certificate (or maybe not an element at all but a setting for the certificate). I need to think about this.
2) Having a look at the code now. It appears the field is only shown if the user currently rearranging the elements has a value for that field. So, if you chose 'Web page' but do not have one yourself then the value will remain as an empty string. This will need to be addressed.
If you could visit https://github.com/markn86/moodle-mod_customcert and create some pull requests to remind me to work on them that would be great!
Regards,
Mark