How to restore Okular default (toolbar) settings

The keyboard shortcut to show/hide the menu bar is Ctrl-m. I found the config file where the setting is stored at ~/.config/okularrc.

So you have to open Okular without any document and right-click on the background surface of where the document is normally shown (for me grey). If -- and only if -- the toolbars are not visible you will provided with a context menu with the option to show the bars.