Hey there!
I’m using some nb_extensions like Collapsible Headingsand binder in classic notebook mode. For that, I added a postBuild file with the following entries:
jupyter contrib nbextension install --user
jupyter nbextension enable codefolding/main
jupyter nbextension enable freeze/main
jupyter nbextension enable collapsible_headings/main
This works fine 
Now I want to customize the collapsible_headings extension so that the toggle icon is removed for uncollapsed cells like it is mentioned in this tutorial. For that, in my local notebook I can simply use the configurator and delete the entry for font-awesome class for uncollapsed headings
Is there any way I can achieve the same by usind Binder to distribute my notebooks to my students? Maybe adding some notebook config file is helpful?