As far as I know, Fortran is not a dead language, but is actively used in the scientific computing & mathematics niche.
> Various NumPy modules use FORTRAN 77 libraries
$ pip show tensorflow theano
Name: tensorflow
Version: 0.7.1
...
Requires: six, protobuf, wheel, numpy
---
Name: Theano
Version: 0.8.2
...
Requires: six, scipy, numpy
You are right that arithmetic in both of them is not written on Fortran, but the packages do use NumPy as you can see in the pip output.