Its something completely custom. It does not have package manager.

Do you have Qt configured to use that display ?

Figured it out.
The 'mark as solved' option is there for people with admin rights, as the functionality relies on the admin 'rename post' function.

For a thread OP to mark a post, he/she needs to add [solved] to the title.

I'll take a look at the plugin that adds the thread tool option to see if it could work with normal OP rights too.