ThemeForest FLATBOOTS - phpBB 3.2 [UPDATE 27 December 2018]

Bootstrap is mobile-first front end framework developed by a couple of engineers at Twitter (hence the name), it was originally designed to help provide some consistency across tools the guys were building internally. Since its first public (open source) release in 2011 (see the blog post here) it has found huge popularity among web developers for its ease of use and feature set, and in 2012 was the most popular project on GitHub. that allows for developing websites and custom applications that are cross-browser and cross-device compatible (responsive). Some of the key features of Twitter Bootstrap are:

Responsive design Base CSS For Typography, code, Tables, Forms, Buttons
Web UI Component Dropdowns, Buttons, Navigation menu, Labels, Breadrumbs, Pagination, Thumbnails, Alerts, Progress bars
JavaScript plugins Modal, Dropdown, Scrollspy, Tab, Tooltip, Popover, Alert, Button, Collapse, Carousel and Typehead
Built on LESS A CSS grid system for designing adaptive websites
Cross platform Ensures that the output is consistent across platforms & browsers
Integration Simple, fast and easy to work with existing sites
Compatibility Takes into account the future design & development standards e.g. HTML5 & CSS3
Open Source Allows developers to participate and add their contribution to be used by other bootstrap developers