From your screenshot, it looks like you are using the default expandedBackgroundColor of #222222 and opacity 1.
If a default value is set in JuiceboxBuilder-Pro, then the corresponding option will not be written to the gallery's XML file.
Usually, if an option is not present in the settings, then when the gallery is displayed, Juicebox will use the default value.
However, if an expandedBackgroundColor is not explicity set in your gallery, then the actual expanded background color will be the same as the normal background color.
(This is the only option which does this.)
The solution is to make sure that there is an expandedBackgroundColor option present in your gallery settings.
Either:
(1) Change the expandedBackgroundColor (or its opacity) in the JuiceboxBuilder-Pro interface.
... or:
(2) Manually add the expandedBackgroundColor option to your gallery's settings. Instructions for setting options can be found here.
You could open your gallery's XML file in a plain text editor and add expandedBackgroundColor="rgba(34,34,34,1)" alongside all the other options (as an attribute to the opening <juiceboxgallery> tag) or add the following line in your gallery's embedding code in your HTML page (ensuring that all options are separated by commas):
expandedBackgroundColor: "rgba(34,34,34,1)",