Thanks a 1M . your links are great and too much informative. when I wanted to ask this question I thought it would a too much easy one. but when I now see your answer, I see there is a lot of nice and exciting information behind the “backend”.
Anyway for short, referring to your second link…
"If you want the Keras modules you write to be compatible with both Theano and TensorFlow, you have to write them via the abstract Keras backend API. "