I went to jQuery UI download site https://jqueryui.com/download/all/, I see for each version there is a jQuery-UI zip and a jQuery-UI-Theme zip for download. I found if I only download jQuery-UI it works in my code along with jQuery that I already have. I think the theme zip is for people who want to specify certain theme in their code and in that case the zip provides the specific .css for that theme. But if I don't download the theme part, does that mean I am using default theme jQuery UI provides ? plus, I know there a customizing jQuery-UI site to download a customized version, but I don't really need to customize anything so I just go with the easiest way here downloading the standard jQuery-UI.
I actually have no answer for your question, but the under "API Documentation" on top of the download page you can find a link to "JQuery Learning Center and from there I found the following urls that may contain an answer for you.