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The easiest and most direct way to specify what theme to use on a page which uses jquery ui is to include the theme css directly, as pasted below.
(taken from the view source from the jquery ui demos page)
<link rel="stylesheet" href="http://ajax.googleapis.com/ajax/libs/jqueryui/1.7.2/themes/ui-lightness/jquery-ui.css" type="text/css" media="all" />
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