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:

Demo: https://themeforest.net/item/flatboots-phpbb-32-highperformance-and-creative-modern-forum-for-phpbb/8536771