Source (required if based on other people's work): https://github.com/trustable-code/Xfce-Simple-Dark
Simple Dark is a dark theme for Xfce. It contains styles for widgets and window titles.
The GTK 2.x and GTK 3.0 widget theme is based on https://github.com/vkuzel/Xfce-darkness .
The GTK 3.20 widget theme is based on https://github.com/horst3180/vertex-theme .
The window title theme is based on Makulu Black.
1. Copy theme folder `Xfce-Simple-Dark` into your `~/.themes/` directory.
2. Open *Appearance* and select the theme.
3. Open *Window Manager* and select the theme.