(Firefox) Non-FVD Theme Backgrounds

So the problem I'm running into is that I will install a non-FVD theme to Firefox and only the title section changes - the background where the FVD Speed Dials are does not change.  There is a YouTube video from 2013 showing how to change the theme in FVD but times (and the add-on) have changed and it's no longer an option.

I know you can select your own image but what if I want to use the background image that comes with another theme?  Is there a place to find this image so it can be incorporated?