@jsulm Sorry for the late response. Thank you very much for very clear answer. It helps.

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.