The Xfce Evolution theme attempts to rid the Xfce desktop of visual distractions by providing a consistent look (buttons, scrollbars, menus, etc) across older and newer applications (Gtk2/3, Qt4/5, wxWidgets2/3).
Effective October 2019, maintenance for this theme has been discontinued; the followup theme is Xfce ClassicLooks
Based on Ambiance gtk2, Adwaita gtk3, and Greybird xfwm4.
Developed and tested on Xubuntu 17.04, 18.04 LTS
Maintained at http://xfce-evolution.sourceforge.net