[annnnd]

I respectfully disagree. Of course you need to really understand backpropagation for any advanced stuff, but it is easier to ignore it at the beginning. Take Keras container, copy some MNIST example from somewhere and tweak it. Then, when you have a general feel of how training NNs works, gradually learn about each concept - BP should of course be one of the first. By the time you get into math stuff it will probably make much more sense because you will understand how it applies to your case.

But I guess the approach depends on how you best learn, so there is no wrong answer. Just jump in!