This method is based on google, and people who don't know how to set up a robots.txt
properly/don't care/don't even know what it is. So here is the thing: if you don't set up your
robots.txt properly, google might index pages that you don't want it to. This includes the wp-content/uploads directory where uploaded themes and plugins are stored.
All we need to do is search for these pages.
Go to google, and input the following line:
(htm|html|php)intitle:”index of" + “Newsmag” (zip|rar)
Obviously, "Newsmag" is the name of the theme you are looking for, I just used Foodpress
theme as an example. This will not work with themes that gives its zip file random names
like "abcdefg", since you can't guess it. You always have to guess the name of the zip file, but
in a lot of cases it is identical or a longer/shorter version of the theme's name. In some cases
finding the theme isn't enough, because the theme requires a serial key or ID&PW to work. In
these cases, you can't use the theme. If you are pro in PHP you can try to break their protection
though. (same is the case with plugins.)