The arboreal theory of bird evolution ("trees down") is held by a number of researchers and academics. The core of the theory is that the lineage leading to birds began in the trees (not on the ground).
The pterosaur to bird theory is a different idea. Pterosaurs were already flying in the trees, by flapping their skin membrane, and they later developed feathers.
The ancestor of birds was not a dinosaur, nor was it a thecodont. It was a pterosaur.
The trees down theory begins with a gliding arboreal creature. This is closer to being correct except that the gliding phase took place earlier, in the evolution of the pterosaur. This is a key point.