In your http://www.reisedampfer.de/html5/norwegen_2014/ page, your gallery's embedding code uses the following line:
backgroundColor: 'rgba(204,204,204,1)'However, when you embed the gallery in your http://www.reisedampfer.de/app595898963 … wegen-2014 page, the embedding code uses the following line (resulting in a white background):
backgroundColor: "rgba(255,255,255,1)"Change the backgroundColor in the embedding code to either rgba(204,204,204,1) (to match the background color of your page) or rgba(0,0,0,0) (for the gallery's background to be fully transparent so that your own page's background shines through).
If you want to set a different color for the expanded gallery, use the expandedBackgroundColor configuration option (in JuiceboxBuilder-Pro's 'Customize -> Color' section).
Your gallery does not currently use an expandedBackgroundColor so the expanded background color will be the same as the normal (unexpanded) background color.