Using Clean theme with top drop-down menu 'Invert navbar' setting checked. If not already added proceed to Appearance > Themes > Theme settings and add the default example 'Custom menu'. Float the browser window and decrease the width until the top menu hides all the menu items and leaves a 'hamburger' icon top right next to the User Menu. Verify that after clicking on the icon the horizontal divider lines in the 'Custom menu' are a light grey and not white.