Why isn't the background image included?
Upgrade to Carbonize v1.1 to fix this issue.
Download and install Carbonize v1.1 from the Mac App Store to fix this issue and enable instant exports.
Carbon.now.sh uses two distinct ways to generate an image from code. On Chrome it uses the browser itself. For Safari (which is what Carbonize uses) the image gets generated on a server through an API call. It seems the custom theme feature has not been implemented yet on the backend. This means it won’t work in Carbonize. While developing Carbonize I was on the fence about either hiding these types of (currently) unsupported features, or leaving them in. I chose to leave them in for now, so that if these option do get implemented, Carbonize would be ready instantly without me having to release an update.