They have their own theme that they designed. Unless you recognize the theme elements, there is no way to know if they based it on someone else’s theme or not, but most reputable sites credit any design work to the designer in the footer. One way to find out if someone is using a particular theme is to view the page source and look at the path of the stylesheet. For example, wpcandy.com has the following line:
<link rel="stylesheet" href="http://wpcandy.com/wp-content/themes/wpcandy3/style.css" type="text/css" media="screen" />
The theme name they are using is ‘wpcandy3’. Many advanced sites use their own themes that aren’t published for anyone else to use. wpcandy.com is one good example of such a site.
When I design themes for private use, I never base it on someone else’s theme. I always design the theme from scratch. I would suspect that wpcandy.com is the same.
