Autumn Light has a cutout edge on the "Leave a Comment" button. It appears that the button edge background is transparent, but because of the variable positioning of the button, it's not possible to set up the button as a transparent.
If you change the overall background of the site, you'll need to take a small piece of the background you choose and place it behind this area on the "leave a comment" button. Using the techniques you learn in my video tutorials to isolate the background piece in the "leave a comment" group (v, command, double click).
Hide the background cut out layer and replace it with a piece of your background, then export your new graphic. The new graphic will be uploaded to ProPhoto 4 --> Customize --> Comments --> Comments Header --> Add a comment link.