The Vimix theme suite provides flat themes for your Ubuntu 13.10 system. Here's how you can install them.
1. Open a terminal window.
2. Type in the following commands then hit Enter after each.
sudo add-apt-repository ppa:noobslab/themes
sudo apt-get update
sudo apt-get install vimix-flat-themes
This pack has ten theme variants which contains round corners and dark panes. You can use Unity Tweak Tool, Gnome-tweak-tool or Ubuntu-Tweak to change themes/icons.More at Noobs Lab
Additional resources for Vimix Flat Themes