I followed the instructions in this post and they worked like a charm... for Python 2. But not for Python 3.
I'm running Anaconda on Windows. To activate Python 2 from the command prompt I type
activate python2
So, when I type
jupyter notebook
after having activated Python 2, the Jupyter notebook that pops up is affected by my custom.css file (and it runs Python 2). But if I start the Jupyter notebook without having activated Python 2 (which results in the Jupyter Notebook running Python 3), the notebook looks the default way. What can I do?