If you were a Juicebox-Pro user, you could use the imagePadding and/or stagePadding configuration options to add padding around a main image when no thumbnails are being displayed. Both configuration options are Juicebox-Pro only (and are not available to Juicebox-Lite users) and take effect when thumbnails are displayed or hidden. The stagePadding surrounds the entire gallery (including thumbnails when they are displayed) and the imagePadding surrounds only the main image.

If using Juicebox-Lite, perhaps the easiest way to create padding around an entire gallery would be to create a border (of the same color as your gallery's background) around your gallery container <div> using CSS.

It would not be possible to create padding around a main image only when thumbnails are hidden by editing a Juicebox-Lite gallery's 'theme.css' file as the logic behind determining whether or not thumbnails are hidden is coded within the 'juicebox.js' JavaScript file which is obfuscated and cannot be modified.