It works by pulling all data from the site and scouring the stylesheet. Most WordPress themes include links to the original theme page & the developer’s web page in the comments.
Rather than going through all this work yourself, you can let this site do all the heavy-lifting.
While testing this web app, you’ll notice that many sites use their own themes and simply do not have options to browse. Hongkiat.com is one example where the theme is so heavily customized that you won’t get any data.
Good news is that it’s a pretty simple app to use. Everything is powered by Ajax, so you just punch in the URL and let it load.
For some sites, you’ll even find out what plugins they’re using. This is very helpful since you can track down how certain websites made their sticky sidebars, custom tables, or any other features you may want to replicate.
If a website uses a custom theme you’ll get a message saying it does run WordPress but the theme can’t be found.
And, if it’s using any buyable (or free) theme you’ll get all the related info:
Theme purchase page
You’ll also get some details for all the installed plugins. How this searches all the plugins I have no idea. But, it’s really cool!
You can find a bit more info on the FAQ page if you’re curious. Overall, this app is phenomenal and surprisingly accurate. If you’re ever curious or desperate to locate what theme a site is running it will absolutely help.