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Unable to access "open" nav menu state

I recently encountered unexpected behavior of the Navbar element.
I am trying to make the fullscreen mobile menu. For that, I need to change style of nav menu “open” state. When I am pressing the “open menu” button in the navbar settings tab, the menu opens as it should, but the green “Open” state class does not show on in the “selector” field and all the styling I apply to Nav Menu doesn’t work at all.

How can I access its “open state” or style it in some other way?

Thank You for your answers!

Here is my site Read-Only:LINK