What is this new cat in the box theory about?

That's why i do prefer dogs : no doubts about them 😉

There are no theories in this.  The original proposal was four philosophical proposals of what MIGHT have happened during a time that we cannot know.  Because they are not testable they cannot be a theory or even an hypothesis.  I cannot know which of these you are asking about.

I suspect you’re talking about the Wigner’s friend paradox. It’s a similar idea to Schrödinger’s cat but implies that a measurement doesn’t create a classical reality that everyone agrees with. It’s not really new, it’s been around awhile, but newly resurrected...

I’ll use the cat as an example, but the Wigner’s scenario is different and quite a bit more complicated ...So, basically, if Wigner’s friend opens Schrödinger’s cat box, the cat either being alive or dead is considered objective reality to Wigner’s friend. It’s classical information and everybody should agree on that. Let’s say his friend finds a live cat.However, If Wigner acts as an observer of his friend, he can create a measurement scenario that results in a dead cat. So, his friend sees a live cat, Wigner sees a dead cat. Contradictory views of classical reality. Lots of sticky issues.

It is not New.
google "Schrodinger's Cat's and read it.

No.  Nobody wants to explain it to you and nobody will explain it to you.