sub-menu items separation

Hello, searching among old posts i found a way to put a separator into sub-menu items (desktop version):

.mainnav .menu .sub-menu li {
border-bottom: 2px solid #ffffff !important;
}

but i found it was not working (I tried to insert it into child theme’s style.css).

Inspecting the page, i noticed that the classes for submenu items <li> are:

class=”menu-item menu-item-type-post_type menu-item-object-page menu-item-###” (where ### is the menu item ID).

i tried then to substitute those classes to the css code, but it did not work.

what should i do then? thanks.

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