Comment 4 for bug 1910438

The menu.css attached is the default one, so it makes sense it doesn't change anything
however, you may be able to modify it to maybe solve the problem:

Apparently the hovered item as a background color; so it's only the menu background.
so in this part:
.gldimenu {
        background-color: @menu_bg_color;
        background-image: none;
        color: @menuitem_text_color;
}