Inverness is a fairly dark and fairly flat theme that works extremely well in Linux Mint 19.1 and Ubuntu 18.04.
Gnome shell included.
To install, use Gnome tweak tool and add Gnome extensions.
Mint Menu icon is included.
Tested with Mint19.1/Ubuntu 18.04
Supports: Gnome, Unity, XFCE and Openbox.
The Gnome icons set may be downloaded @
For convenience, individual colors may be downloaded here.
Available Colors: Gorm (blue), Uaine (green), and Ruadh (red)