Just create a style similar to the one below and use the dialogClass parameter in those dialogs that you want to have a transparent background. Of course, you can create multiple styles and convey whatever you want.
<style type="text/css" media="screen"> .transparent { background:transparent } </style> //make dialog with transparent background $("#dialog").dialog({dialogClass:'transparent'}); //make default dialog $("#dialog2").dialog();
Check out the demo site: jsBin (base jquery, jquery ui, jquery ui css + custom css transparency class)
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