i've been having problems with xfce where themes with rounded corners show a black box behind the whisker menu and even with the following code in ~/.config/gtk-3.0/gtk.css

}#whiskermenu-window { border-radius: 10px; border: none; box-shadow: none; border-image: none; background-image: none; outline: none; }

searches on duckduckgo returned nothing but after a few months with this problem i found a solution on google on the zorin os forums

the modifications: first, in the gtk.css file: __#whiskermenu-window frame>border { border-radius: 10px; border: none; box-shadow: none; border-image: none; background-image: none; outline: none; }_

then in the whisker menu properties, turn the opactiy to 99

no idea how or why this works but it just works :)