Ruairi, thank you for replying.
The alternative menu in the sidebar is not the easiest approach for me because I've chosen to have a somewhat multicoloured background image for the main site's background which sometimes makes the text on the sidebar hardly readable. Besides, I like the appearance of multi-level menu in Sorbet on its own account even without toggling and I am afraid that users that are not savvy enough to know what the three line menu icon does, will not be able find out about this nice looking menu at all.
Anyway, I think I've found a way to show the primary menu by default without losing the toggle functionality.
In child theme's style.css I put:
This makes the menu always on.
Also, you can make the menu toggle button to light up by default. Open sorbet/js/menu.js and add the following line before “// Display/hide sidebar”
I do not know, though, how to modify this code with a child theme.