I'm running into kind of a consistent, recurring problem with my plugin and other plugins including JQuery UI.
My plugin displays dialogs, so I enqueue the appropriate javascript and wp-dialog CSS files.
However other plugins are including their own copy of JQuery-UI.css which causes the dialog to display incorrectly. You can see and example here:
http://wickfordcatholic.com/anniversaries/
click on one of the anniversary names. Here's another thread where I worked with another plugin vendor:
https://wordpress.org/support/topic/breaks-dialogs-includes-jquery-ui?replies=3
I understand why people are doing this. I tried to use the JQuery UI Datepicker, and was stymied because WP doesn't include the required CSS to make it work. That was super-frustrating because WP explictly names out the jquery-ui-datepicker as a script to be loaded.
I tried loading my own copy of JQuery UI.css from a CDN and I ran into the same problem.
I understand the issue with including a JQuery-UI theme because it might not match the WP theme at all, making people really unhappy.
Is this kind of problem solvable? I'm more of a back-end developer. Could a system be created to dynamically generate a complete JQuery-UI css file using hints provided by the theme? That way a themed, functionally complete JQuery UI CSS file could be delivered?