EDIT: Here’s a solution that’s suitable for all kinds of backgrounds/wallpapers, not just dark ones.
The steps are as follows:
Composite (in xfwm settings) must be activated and opacity must be lowered to at least 99% within whisker menu > properties for this to work!
Create and edit this .css file from terminal:
nano ~/.config/gtk-3.0/gtk.css
Paste this into it:
#whiskermenu-window {
background-color: rgba(0,0,0,0.7);
color: #ccc;
}
#whiskermenu-window button {
background-color: transparent;
color: #ccc;
}
#whiskermenu-window button:hover {
background-color: transparent;
color: #fff;
}
#whiskermenu-window button:checked {
background-color: transparent;
color: #fff;
}
#whiskermenu-window treeview {
background-color: transparent;
color: #ccc;
}
#whiskermenu-window treeview:hover {
background-color: transparent;
color: #fff;
}
Save the file by pressing:
Strg+O
(Confirm with ENTER), then exit nano:
Strg+X
Finally restart whisker menu:
xfce4-panel -r
Enjoy!
Wallpaper by @Lunix
(The resulting look from the code posted above differs slightly from the whisker menu in the screenshot in order to make it work with brighter backgrounds as well)