The Google AJAX Libraries API, which includes jQuery UI (currently v1.10.3), also includes popular themes as per the jQuery UI blog:
Google Ajax Libraries API (CDN)
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Uncompressed: http://ajax.googleapis.com/ajax/libs/jqueryui/1.10.3/jquery-ui.js
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Compressed: http://ajax.googleapis.com/ajax/libs/jqueryui/1.10.3/jquery-ui.min.js
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Themes Uncompressed:
black-tie, blitzer, cupertino, dark-hive, dot-luv, eggplant, excite-bike, flick, hot-sneaks, humanity, le-frog, mint-choc, overcast,pepper-grinder, redmond, smoothness, south-street, start, sunny, swanky-purse, trontastic, ui-darkness, ui-lightness, and vader. -
Themes Compressed:
black-tie, blitzer, cupertino, dark-hive, dot-luv, eggplant, excite-bike, flick, hot-sneaks, humanity, le-frog, mint-choc, overcast,pepper-grinder, redmond, smoothness, south-street, start, sunny, swanky-purse, trontastic, ui-darkness, ui-lightness, and vader.