Whisker menu is a nice, customisable menu/launcher type thing mainly used in XFCE. My main problem with it is that it draws itself as an app window, so isn’t affected in the same way as actual menus by XFCE themes.

This site has some information on the CSS needed to customise it. I’m not going to cover the same ground, except to say you need to put your custom CSS in ~/.config/gtk-3.0/gtk.css and run xfce4-panel -r afterwards.

Here’s mine. Suck it and see.

whiskermenu-window {
background-color: #404040;
color: #ccc;
}
/* Make sidebar buttons match */
whiskermenu-window button {
background-color: #404040;
color: #ccc;
}
whiskermenu-window button:hover {
background-color: #808080;
color: #000;
}
whiskermenu-window button:checked {
background-color: #808080;
color: #000;
}
/* Make treeview match */
whiskermenu-window treeview {
background-color: #959595;
color: #222;
}
whiskermenu-window treeview:hover {
background-color: #404040;
color: #fff;
}
whiskermenu-window border{
border-color: #444444;
color: #fafafa; 
}