jQuery Dialog theme not being applied

I'm using jquery-1.4.2.min.js and jquery-ui-1.8.1.custom.min.js. According to Firebug, both minimized scripts have been loaded. They both appear in the dropdown list on Firebug's Scripts tab.

The dialog is launching, but there's no background and so the text below it shows through, and there's no "window" stuff at all, no titlebar, no X button. Just the floating/draggable text with a -close- l开发者_如何学Cink at the top left corner.

I was under the impression from reading these pages

 http://jqueryui.com/demos/dialog/
 http://jqueryui.com/themeroller/

that the UI theme-driven appearance would be automatically applied to the dialog when it was invoked using this barebones call:

 $('#dialog').dialog()

I would be grateful for suggestions on how to troubleshoot this. Thanks

You need to make sure you include the link to the jQuery ui theme

<link href="http://ajax.googleapis.com/ajax/libs/jqueryui/1.8/themes/base/jquery-ui.css" rel="stylesheet" type="text/css"/>