Creating a custom theme with "Submit" button in Jupyter Lab for Binderhub


I am trying to build a theme for use with Binderhub/Jupyter Lab that has a submit button.
When users launch binderhub, they input their Github url. Then, the Jupyter Lab should launch with
a custom theme that has a submit button. The user can edit their work in Jupyter Lab and once they are ready to submit, they will click the “Submit” button. All the files will be submitted as a POST request to a private server.

I was wondering if anybody had experience with customizing JupyterLab or knows the way to do this?

Any help or pointing in the right direction would be appreciated.