You can already do that with the existing XenForo node permissions. You set the Admin / Mod team to be able to view all threads, and revoke the permission to view threads by others for everyone else.
That's how I run my own existing support section, which I want to replace with the ticket system.
After the latest update, as there been a change to the message background colour?
I'm seeing the very light background colour, but I can't for the life of me figure where in the SP's this is being set.