I did a post a while back about customising the XFCE Whisker menu. I’ve expanded on this and rolled the customisations into a theme.
The theme is based on ARC-Darker. By based on, I mean it is almost a 100% copy. The only parts I’ve changed are the colours for Whisker menu and a slight tweak to the context menu to make it more visible on light backgrounds. I find the theme looks nicest with the ePapirus icon theme, but YMMV.
To install, download this file
Extract it and copy to its own folder in either /usr/share/themes or ~/.themes
If you previously customised whisker menu as per my previous post, you will need to remove or rename ~/config/gtk-3.0/gtk.css and run xfce4-panel -r to reload and reset the panel config.