I’ll simplify this – can I change the default block theme fixed navigation
The navigation in the default theme is just an instance of the standard Navigation block placed inside the header template. There’s nothing special about it.
… to side panel (dashboard style)?
You can place the same navigation block used in the header anywhere else you want.
But I’m not entirely sure what you mean by “side panel (dashboard style)”, considering you said earlier you’re not concerned about vertical vs horizontal orientation.
But I see four basic things in the admin panel’s left navigation menu.
1) Vertical main menu
2) Vertical sub-menus
3) Collapsible width
4) Styling (dark theme)
Anything else I missed?
Other than item (3) which may require a bit of JavaScript or custom CSS code to do, the other three are easily done in the site editor without writing any code.
I’ve I misunderstood your question please explain further with either an annotated screenshot and/or a “user story” (when you do X, Y happens) sort of description.
Standing by.