Please see picture attached for better understanding.

The point was to change a style for <div class="float-break" id="topmenu"> present in a standard ezpublish design.
The topmenu element zone (number 1 on the picture) is tightly covered by a zone of path element (number 2 on the picture) that makes it impossible to choose. It is also not accessible via style editor top menu.

The only way to change the style properties for that DIV element, was to disable the path element zone via firebug.