For anyone reading this thread and wanting to change the caption font size, please see this post for details of how it can be achieved.
However, every time I need to update the gallery and publish/save it, the style sheet is rewritten and the additional code is gone. Anyway to get around this?
You could add the CSS code to one of the template files that JuiceboxBuilder-Pro uses to create the gallery.
For example, you could add the CSS code to the template 'theme.css' file in the following location on a Windows 10 PC. (If you use a different operating system, just search for the 'JuiceboxBuilder-Pro\template' folder.)
C:\Program Files (x86)\JuiceboxBuilder-Pro\template\jbcore\classic
Alternatively, if you use the gallery 'index.html' files, you could add the CSS code (within <style> ... </style> tags) to the template 'index.html' file in the following location.
C:\Program Files (x86)\JuiceboxBuilder-Pro\template\index.html
Whatever modifications you make to the files in the 'JuiceboxBuilder-Pro\template' folder will be carried across to the gallery folders on publishing in JuiceboxBuilder-Pro.
Just be aware that these modifications will be overwritten if or when you reinstall or upgrade JuiceboxBuillder-Pro so please be sure to make a backup of your modifications in case you need to reinstate them at a later date.