There are many dark themes for GTK with a simple and good color scheme. But, I have been looking for a simple dark theme especially for Ubuntu 18.04 LTS (Bionic Beaver). I tried many Dark themes on Ubuntu 18.04 LTS (Bionic Beaver) and my mind was set on Qogir Dark theme.
The simple design and the comfort of the dark colors scheme is quite amazing and gives a relief looking for the desktop environment. Qogir comes with a Dark and Light Theme for GTK 2.0 / GTK 3.0 and Gnome Shell. The Dark or the Light theme integration with the default installed applications such as Nautilus file manager, LibreOffice and Mozilla Firefox are quite good.
Make sure you have installed the required dependencies for GTK2 if you are going to use the theme on other Linux distributions.The theme supports GTK+ 3.20 or later.
For a consistent looking with theme it’s highly recommended to use the Arc Icon theme.