The correct way is to say: я дома.

If you want to say that you are in the building - я в доме if you want to say that you are in the place where you are live (you can be in your garden for example) - я дома.

must be "я в домe" or "я дома"

Yes, "я в театре".

So please what of I'm in the theatre? Will it be 'я в театре?'