They are listed on the jQueryUI blog and updated with every release. Both Google and Microsoft provide CDN hosting for the library and themes.
In general, the URL of each theme CSS file is:
// Google
http://ajax.googleapis.com/ajax/libs/jqueryui/[UI.VERSION]/themes/[THEME-NAME]/jquery-ui.css
// Microsoft
http://ajax.aspnetcdn.com/ajax/jquery.ui/[UI.VERSION]/themes/[THEME-NAME]/jquery-ui.css
I have listed the current (UI version 1.8.10) themes below:
- base: Google CDN, Microsoft CDN
- black-tie: Google CDN, Microsoft CDN
- blitzer: Google CDN, Microsoft CDN
- cupertino: Google CDN, Microsoft CDN
- dark-hive: Google CDN, Microsoft CDN
- dot-luv: Google CDN, Microsoft CDN
- eggplant: Google CDN, Microsoft CDN
- excite-bike: Google CDN, Microsoft CDN
- flick: Google CDN, Microsoft CDN
- hot-sneaks: Google CDN, Microsoft CDN
- humanity: Google CDN, Microsoft CDN
- le-frog: Google CDN, Microsoft CDN
- mint-choc: Google CDN, Microsoft CDN
- overcast: Google CDN, Microsoft CDN
- pepper-grinder: Google CDN, Microsoft CDN
- redmond: Google CDN, Microsoft CDN
- smoothness: Google CDN, Microsoft CDN
- south-street: Google CDN, Microsoft CDN
- start: Google CDN, Microsoft CDN
- sunny: Google CDN, Microsoft CDN
- swanky-purse: Google CDN, Microsoft CDN
- trontastic: Google CDN, Microsoft CDN
- ui-darkness: Google CDN, Microsoft CDN
- ui-lightness: Google CDN, Microsoft CDN
- vader: Google CDN, Microsoft CDN
http://www.the-xavi.com/articles/jquery-ui-css-themes-hosted-on-cdn