So when I delete that file, it will recreate itself once I make changes on an individual page, I assume?
If you delete the 'showkase/_data/themedata/boma/juicebox.xml' file, it will be recreated when you change the default settings in the 'Site -> Customize Viewers -> Juicebox-Pro' section.
If I set up another site, can I copy that same file I'd be deleting above, and add it to that same location so the new site would have the same values for the galleries?
You should be able to do that. As long as the versions of Showkase and Juicebox-Pro match, I expect it would work (but it would not be officially supported).
You are certainly free to try whatever you like but I would strongly advise that you work within the Showkase interface, within the realms of what Showkase was designed to do.
If Showkase ever changes the way things are handled internally, then this method might break, leaving you with a workflow that you may be used to but which no longer functions as you expect it to.
what is this list of NOTICE: Undefined Offset (with a number) ?
I would need more information to troubleshoot this. Please post the exact notice the next time you see it and let me know exactly what you were doing (or trying to do) what it happened.
As this is a notice (rather than a warning or an error), it is unlikely to actually affect your site but you should not be seeing such notices and it would be good to figure out where they are coming from. Thank you.