Overlay/Transparent Menu

Overlay/Transparent on Desktop

Go to Headers custom post types, edit your header, scroll down to Liquid Header Options and choose the Overlay option.

Overlay/Transparent Mobile

Overlay menu can be also used for the mobile menu. 

Go to Headers > Your Header > Liquid Header Options > Mobile Navigation. Select these options:

Open Header Background option and drop the Alpha (Opacity) value to 0.

This is how it should look now: