Option for custom styling of spawning progress bar

[stevegore]

It would be great to have an option for custom styling of the progress bar users see when an instance of Jupyter is spawning for them.

Jupyter itself we can customise using custom.css and custom.js, but the spawning progress screen doesn't seem to have an equivalent. Am I missing something?

Not sure this is relevant but I'm using Zero to Jupyterhub specifically.