I use jQuery quite extensively in my projects these days, but I kind of ignored jQuery UI for too long. The last two days, though, I finally got more into jQuery UI, after I was looking for a modal dialog component and realized that jQuery UI provides a very nice implementation by itself.
I haven't done much directly with the available components (except the dialog component), but
I was very much impressed about the way you download jQuery UI. They call it "Build Your Download" which let's you pick and choose which components of jQuery UI you may want to use and it creates a customized download bundle for you.
But substantially more impressive is "ThemeRoller", which not only let's you select between various themes but also allows for editing and creating new themes. All changes are applied right on the spot and once you're done you can include the theme in your customized jQuery UI download.
From there I discovered that jQuery released a new CSS Framework called "jQuery UI CSS Framework". It looks quite promising and I wonder if it might be applicable for my pet project. I certainly need to learn more about it...it would be nice if it could serve as the main CSS for my web application and therefore allow for easy theming of the application.
Anyway the docs can be found at http://docs.jquery.com/UI/Theming/API and the wiki pages are located at http://jqueryui.pbwiki.com/jQuery-UI-CSS-Framework
There is also a themeswitcher component available.
Lastly there is an interesting article on the blog of filament group, one of the main sponsors of jQuery UI, about styling buttons and toobars.