I am not sure exactly what you are trying to do but you can style captions using HTML tags within the captions themselves.
For example, you could enter a caption such as the following into an image's caption field in the 'Images' tab of JuiceboxBuilder-Pro to achieve a multi-line caption with different indentations for each line.

<span style="margin: 20px;">Line 1</span><br /><span style="margin: 40px;">Line 2</span>

There are also configuration options available (such as maxCaptionHeight) which might help with your scenario (depending on where in your gallery your captions are positioned).

Otherwise, if you wish to modify the 'jbcore/classic/theme.css' file, the section which deals with the gallery caption area can be found starting at line 193 of the Juicebox-Pro v1.2.0 version.