How can you enable GTK3 themes' dark theme mode when using Unity 7?

You can enable 'Global Dark Theme' mode on Unity 7, the same way it's enabled on Gnome Shell. Just install GNOME Tweak Tool by clicking here:

Or run this in Terminal:

sudo apt install gnome-tweak-tool

When it's installed, open Tweak Tool from the Dash and toggle on 'Global Dark Theme' switch as shown below:

From now on you can use the dark mode of themes under Unity 7 the same way you use them on GNOME!😊 If it's not working, just check if the theme implements GNOME's dark theme mode.

Manual Method

GNOME Tweak Tool and most of the options it provides are tailored for Gnome Shell and so may not work as intended on Unity. Coz of this, many users prefer not to install GNOME Tweak Tool and choose Unity Tweak Tool over it. If for any reason you don't wanna install GNOME Tweak Tool, this is how you can enable the 'Global Dark Theme' mode manually:

Step 1: Open the GTK3 configuration file in Text Editor:

gedit ~/.config/gtk-3.0/settings.ini

Step 2: Paste this into the file opened by Text Editor:

[Settings]
gtk-application-prefer-dark-theme=1

and save it. That's all! Enjoy your dark themes 😃