If you do not want to have to modify the 'theme.css' file each time you update a gallery, you can either:
(1) Modify the 'theme.css' template file used by JuiceboxBuilder-Pro to create each gallery's own 'theme.css' file.
On a Windows PC, the file will be located here: C:\Program Files (x86)\JuiceboxBuilder-Pro\template\jbcore\classic\theme.css
... or:
(2) Add the following CSS code to the <head> section of the web page into which you are embedding your gallery:
<style type="text/css">
.jb-caption .jb-caption-title {
font-size: 40px !important;
}
.jb-caption p {
font-size: 20px !important;
}
</style>