Looking for Completely Flat Design Theme, Install Ultra-Flat Theme in Ubuntu 15.10/15.04

Are you fan of flat themes? If yes, this theme is designed for you. Basically this theme is modified version of numix theme, without borders, flat rounded window buttons and grey selection color. It is now compatible with Gtk 3.16 and GTK 3.14, if you experience any kind of issue while using it then report to get it fixed. This theme can work under most of the desktop environments such as Unity, Gnome Shell, Gnome Classic and fallback, Cinnamon, Mate, and others. Square icons used in following screenshots. You can use Unity Tweak Tool, Gnome-tweak-tool or Ubuntu-Tweak to change themes.

Available for Ubuntu 15.10 Wily/15.04 Vivid/other related Ubuntu derivatives
To install Ultra-Flat theme in Ubuntu/Linux Mint open Terminal (Press Ctrl+Alt+T) and copy the following commands in the Terminal:

Terminal Commands:
sudo add-apt-repository ppa:noobslab/themes
sudo apt-get update
sudo apt-get install ultra-flat-theme

To fix software center text issue enter these commands in terminal:

Terminal Commands:
cd /usr/share/software-center/ui/gtk3/css/ && sudo mv softwarecenter.css softwarecenter.css.backup
cd /usr/share/software-center/ui/gtk3/art/ && sudo mv stipple.png stipple.png.backup
cd && wget -O softwarecenter.css http://drive.noobslab.com/data/themes/delorean-dark-3.10/softwarecenter.css
sudo mv softwarecenter.css /usr/share/software-center/ui/gtk3/css/
cd && wget -O stipple.png http://drive.noobslab.com/data/themes/delorean-dark-3.10/stipple.png
sudo mv stipple.png /usr/share/software-center/ui/gtk3/art/

To revert back software center theme simply enter these commands:

Terminal Commands:
cd /usr/share/software-center/ui/gtk3/css/ && sudo mv softwarecenter.css.backup softwarecenter.css
cd /usr/share/software-center/ui/gtk3/art/ && sudo mv stipple.png.backup stipple.png

That's it