Bootstrap, the popular front-end framework which hailed from Twitter, continues to grow as a quick way for developers to get an attractive site up and running in little time. It’s a great starting point, but Bootstrap is often left completely un-customized, and its popularity has left the framework’s distinct design looking default and monotonous. To bring some variety to this space, Designmodo, the team behind Dribbbleboard and this slick IMDB concept, is launching Flat UI, a free HTML user interface kit based on Bootstrap. The kit packs PSD files of buttons, inputs, selects, checkboxes, tags, menus, progress bars, sliders and navigation elements, as well as color swatches and SVG glyphs.