How do I add a full-screen drop-down div beneath the toolbar/menubar?

Have you read through Request for Guidance on Customizing JupyterLab UI Layout yet?

I know that the title says ‘JupyterLab’; however, as spelled out in the section ‘Custom themes in Notebook 7’ of the documentation:

“Creating a custom theme for Notebook 7 follows the same process as creating a custom theme for JupyterLab 4.”

I know that doesn’t really address things either because it sounds like you don’t only want to make a theme, yetI think some aspects are going to overlap.
It sounds like you don’t want just to make a theme, but to make an extension, see:

If you are looking for other extensions to model your specific implementation on, you may like to look at JupyterLab Marketplace (extension catalog): labextensions.dev.



If you were using JupyterLab, there is ipylab that lets you “Control JupyterLab from Python notebooks.”