Mantis is a dark gray theme for Xfce.
It's existence comes from a need for an easy-on-the-eyes workstation theme.
The GTK 2 widget theme is based on Xfce-Simple-Dark (https://github.com/trustable-code/Xfce-Simple-Dark)
Basic GTK3 support exists and is lifted from my dark gray spin of the Arc theme (https://github.com/mantissa-/arc-pro-theme)
Xfce panel theming needs some work, but it's not the worst.
Tested on Xubuntu 17.10, YMMV.