Customize NodeRed Web UI

I wanted to know how big of task is too customize the Node-RED web UI. I have already gone through Editor Themes and Runtime configuration ways that Node-RED provides for cosmetic customization.
https://nodered.org/docs/user-guide/runtime/configuration
So, my understanding is to achieve what I need, I need to make changes to Node-RED git source to come up with solution.

I am more interested in something like (List is big, but for example)

  1. Adding left pane (Left to palette) for site navigation on editor screen.
  2. Adding custom menu options and handle them.
  3. Adding multiple workspaces in one editor.

Can anyone give me few pointers to start off or anyone has tried it, what are the difficulties / or want to share few tips?