I bought a Fukien Tea bonsai two months ago. The leaves weren't bright green and showed a slight yellow. I bought this bonsai bc the shape was so unique and beautiful and thought it would revive. The seller said it was because of the weather, and that it would green up soon. Well, the poor thing has just kept losing its leaves. I had to cut some of the smaller dead twigs off since they were obviously dead (crispy branches, scratch test showed no green...). The tree is about 35 years old.

The dead branches are gray and the knuckles turn black. Can someone help me stop this? What is causing this?