That worked. To change the background color, it needs to be specified in both the config.xml and in the index.html, which was a little counter-intuitive.

I can confirm that this works well. 

If you open the plugin's 'juicebox.lrwebengine/config.xml' file and add your preset's XML options to the opening <juiceboxgallery> tag, each time you create a gallery using the plugin, your options will be used.

There is I believe a small bug that is if you enter html code (e.g. <br>) in the title field in Lightroom the plugin hangs.  This also applies if it is in the presets XML options.

Getting around this bug is the only reason I have to go into JuiceboxBuilder-Pro after creating in Lightroom.

I would agree with another poster that development of more options in Lightroom would be a good selling point (and a great convenience). 

There is a limited number of good configurable web engines for Lightroom.  On a commercial note I believe that buyers of Lightroom are typically people who will pay for good automation.

Unfortunately, this is a known issue for which there is no known fix. However, if you delete the HTML code from the 'Gallery Title' text field, the Live Preview window should return to normal.