HTML without framework is dull. Doing hard-coded CSS and JS are quite difficult with no promising result on cross platform compatibility. So I decided to explore BootStrap as they said it is the most popular web framework.
Available here - http://getbootstrap.com/
Why you need Flat-UI? Seems like a beautiful theme to make my site look professional.
Anyway you could get variety of BootStrap theme out there, feel free to select here http://bootstraphero.com/the-big-badass-list-of-twitter-bootstrap-resources/
Flat-UI is from DesignModo - http://designmodo.com/flat/
What Flat-UI from DesignModo have to offer you?