In terms of complaining, https://who.is/whois/https://fanfic-journal.com gives you some contacts that might be worth trying - they are just the domain name people, though, can't tell who the host is, though.

WordPress won't care because their software looks like a lot of other software. Plus it was at least created with WP (version 3.7) - when you view the source there are a whole load of wp specific locations going on in there. If they've since moved it it would explain why I can't find any of the pages in the root directory that are in mine.

The person who created the theme might well not care either because there is a credit to them on there, although it's bizarrely, not a link, even though it should be (I found other blogs using the same them at http://www.wpbyexample.com/wordpress-theme/TheSearchEngineGuysLLC/Locket).

Edit: Also, WordPress is free (I wouldn't use it otherwise!), so not sure it can be stolen in quite the way you mean.