The current theme designs follow a very interesting trend and many of the new releases are flat, just the like the one used by Windows. Linux designers don’t settle with so little, so the flat themes for the open source platforms are much more interesting. Proof is the Irish Dark theme.You might think that you don’t need or want a flat theme, but you owe it for yourself to try the Iris Dark 1.5 theme.
“Iris Dark is a flat theme that uses varying shades and tones to create distinction and a modern experience. It supports Gtk 3.10, Gtk 2 (using the Murrine engine) as well as Granite Widgets,” said the developer on deviantart.com.
The theme works on an assortment of operating systems, but you will have to install it manually. This means that you need to copy the theme to "/usr/share/themes/."
You will also need to activate it. For this, you can use a tool like GNOME Tweak (which doesn’t work for the latest Ubuntu 14.04) or Unity Tweak Tool.
Iris Dark has been in the beta stages for quite a while, but the designer has finally released the stable version. You can download it from the official website.