custom.css and custom.js

[sherdim]

Jupyter of recent version recommends to set custom notebook tunings in custom files.
For example ~\.jupyter\custom\custom.css in user's local environment is imported as
http://localhost:8888/custom/custom.css

Mybinder deploys those empty files just as in the default config.
Could it be tuned to take custom.css from the submitted root document tree or special custom directory?