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Numix, a modern, flat GTK3 theme, has recently been updated with dark theme assets, along with other tweaks and fixes. The theme is compatible with GTK 3.6 and 3.8.

Numix GTK3 theme
Numix GTK3 theme under Unity

Numix GTK3 is available for GNOME, Unity, Pantheon, Xfce and OpenBox and comes with a matching GTK 2 theme. Besides dark theme assets, the latest version comes with various tweaks to the Metacity and Mutter themes, changed scrollbar style in GNOME Terminal to match the dark theme, redesigned infobars and many other tweaks and fixes.

The dark theme is used by default when using applications such as Eye of GNOME image viewer or Totem. In GNOME Shell (does not work with Unity) you can force all applications to use the dark theme by using GNOME Tweak Tool: Theme > Enable dark theme for all applications:

Numix GTK3 theme
Numix GTK3 - dark theme under GNOME Shell

Here are a few more Numix screenshots:

New infobars

Numix GTK3 theme
Numix GTK3 theme under GNOME Shell

Numix GTK3 theme
Numix GTK3 Widget Factory

Numix GTK3 theme
Numix GTK3 theme under Xfce

Install Numix GTK3 theme in Ubuntu

Numix is available in a PPA for Ubuntu 13.10, 13.04 and 12.10. To add the PPA and install it, use the following commands:
sudo add-apt-repository ppa:satyajit-happy/themes
sudo apt-get update
sudo apt-get install numix-gtk-theme
Arch Linux users can install the latest Numix theme via AUR.

For other Linux distributions, download the theme from DeviantArt, extract it and copy the theme to /usr/share/themes/ .

Once installed, you can use tools like GNOME Tweak Tool, Unsettings, Unity Tweak Tool and so on to apply the theme.

Numix uTouch
Numix uTouch icons; image via mechoslav
For GNOME Shell, there's a nice Numix-uTouch icon theme you can use. To install it in Ubuntu, copy / paste the following commands in a terminal:
sudo add-apt-repository ppa:numix-icon-theme-dev/utouch
sudo apt-get update
sudo apt-get install numix-utouch-icon-theme

And finally, for a matching GNOME Shell theme, you can use Elegance Colors with the Numix preset.

If you want to change the red used by the theme to something else, you can easily do this by using GTK Theme Preferences, a tool developed by +Satya, who's also the main Numix GTK3 theme developer.